diff --git a/Dockerfile b/Dockerfile
index 65ebe04..c25d6ab 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,13 +1,14 @@
 # == base ======================
 FROM buildpack-deps:bookworm AS base
-ENV CACHEBUST=2024-09-06
-RUN apt update
+ENV CACHEBUST=2024-09-17
+RUN useradd -m -u 8000 observable-builder && mkdir /project && \
+    chown 8000:8000 /project && apt update
 
 ENV RUSTUP_HOME=/usr/local/rustup \
     CARGO_HOME=/usr/local/cargo \
     RUST_VERSION=1.81.0 \
-    VIRTUAL_ENV=/var/local/python-venv
-ENV PATH=/usr/local/cargo/bin:$VIRTUAL_ENV/bin:/root/.local/bin:$PATH
+    VIRTUAL_ENV=/home/observable-builder/.local/python-venv
+ENV PATH=/usr/local/cargo/bin:$VIRTUAL_ENV/bin:/home/observable-builder/.local/bin:$PATH
 
 # == node ======================
 FROM base AS node
@@ -32,8 +33,9 @@ RUN --mount=type=cache,target=/var/cache/apt,id=framework-runtime-python \
       python3-wheel \
       python3-dev \
       python3-venv \
-      pipx \
-    && pipx install poetry \
+      pipx
+USER 8000
+RUN pipx install poetry \
     && python3 -m venv $VIRTUAL_ENV
 
 # == R ===========================
@@ -125,3 +127,5 @@ COPY --from=python . .
 COPY --from=r . .
 COPY --from=duckdb . .
 COPY --from=rust . .
+USER 8000:8000
+WORKDIR /project
diff --git a/package.json b/package.json
index fd832c0..8371127 100644
--- a/package.json
+++ b/package.json
@@ -12,9 +12,11 @@
   "devDependencies": {
     "@types/dockerode": "^3.3.28",
     "@types/node": "^20",
+    "@types/tar-fs": "^2.0.4",
     "dockerode": "^4.0.2",
     "glob": "^10.3.12",
     "semver": "^7.6.0",
+    "tar-fs": "^3.0.6",
     "tsx": "^4.7.1",
     "typescript": "^5.4.3"
   }
diff --git a/tests/dataloader-languages.test.ts b/tests/dataloader-languages.test.ts
index 9cfd7d6..80785fa 100644
--- a/tests/dataloader-languages.test.ts
+++ b/tests/dataloader-languages.test.ts
@@ -71,34 +71,21 @@ describe("Dataloader languages", () => {
     });
 
     test(`Poetry can install dependencies in the virtualenv`, async () => {
-      let testDir = await mkdtemp(join(os.tmpdir(), "poetry-test-"));
-      try {
-        // This will install dependencies using Poetry, and then try to run `ls`
-        // in the installed dependency's package. If the package is not
-        // installed here, the `ls` command will exit non-zero and
-        // `runCommandInContainer` will throw.
-        await cp(
-          "./tests/fixtures/poetry-test/pyproject.toml",
-          `${testDir}/pyproject.toml`,
-        );
-        let res = await runCommandInContainer(
-          [
-            "sh",
-            "-c",
-            "poetry install; ls $(poetry env info --path)/lib/python3.11/site-packages/pip_install_test/__init__.py",
-          ],
-          {
-            workingDir: "/poetry-test",
-            mounts: [{ host: testDir, container: "/poetry-test" }],
-          },
-        );
-      } finally {
-        try {
-          await rm(testDir, { recursive: true });
-        } catch {
-          /* ok */
-        }
-      }
+      // This will install dependencies using Poetry, and then try to run `ls`
+      // in the installed dependency's package. If the package is not
+      // installed here, the `ls` command will exit non-zero and
+      // `runCommandInContainer` will throw.
+      let res = await runCommandInContainer(
+        [
+          "sh",
+          "-c",
+          "poetry install; ls $(poetry env info --path)/lib/python3.11/site-packages/pip_install_test/__init__.py",
+        ],
+        {
+          workingDir: "/project/poetry-test",
+          hostContainerDirs: [{ host: "./tests/fixtures/poetry-test", container: "/project/poetry-test" }],
+        },
+      );
     });
   });
 
diff --git a/tests/index.ts b/tests/index.ts
index 4d0360b..cbc268d 100644
--- a/tests/index.ts
+++ b/tests/index.ts
@@ -1,10 +1,9 @@
 import { test, before } from "node:test";
-import { resolve } from "node:path";
 import assert from "node:assert";
 import Dockerode from "dockerode";
 import { Stream } from "node:stream";
 import semverSatisfies from "semver/functions/satisfies";
-import { basename } from "node:path";
+import tar from "tar-fs";
 
 export const IMAGE_TAG = "observablehq/framework-runtime:test";
 
@@ -87,13 +86,23 @@ function ensureDocker() {
 
 before(ensureDocker);
 
+function copyFilesToContainer(
+  dockerContainer: Dockerode.Container,
+  directories: { host: string; container: string }[],
+) {
+  for (const { host, container } of directories) {
+    const tarStream = tar.pack(host);
+    dockerContainer.putArchive(tarStream, { path: container });
+  }
+}
+
 export async function runCommandInContainer(
   command: string[],
   {
-    mounts = [],
-    workingDir = "/",
+    hostContainerDirs = [],
+    workingDir = "/project",
   }: {
-    mounts?: { host: string; container: string}[];
+    hostContainerDirs?: { host: string; container: string}[];
     workingDir?: string;
   } = {},
 ): Promise<{ stdout: string; stderr: string }> {
@@ -102,13 +111,10 @@ export async function runCommandInContainer(
     WorkingDir: workingDir,
     Image: IMAGE_TAG,
     Cmd: command,
-    HostConfig: {
-      Binds: mounts.map(
-        ({ host, container }) =>
-          `${resolve(host)}:${container}`,
-      ),
-    },
   });
+
+  copyFilesToContainer(container, hostContainerDirs);
+
   const stdout = new StringStream();
   const stderr = new StringStream();
   const attach = await container.attach({
diff --git a/yarn.lock b/yarn.lock
index 15fab4c..7049026 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -215,9 +215,11 @@ __metadata:
   dependencies:
     "@types/dockerode": "npm:^3.3.28"
     "@types/node": "npm:^20"
+    "@types/tar-fs": "npm:^2.0.4"
     dockerode: "npm:^4.0.2"
     glob: "npm:^10.3.12"
     semver: "npm:^7.6.0"
+    tar-fs: "npm:^3.0.6"
     tsx: "npm:^4.7.1"
     typescript: "npm:^5.4.3"
   languageName: unknown
@@ -287,6 +289,25 @@ __metadata:
   languageName: node
   linkType: hard
 
+"@types/tar-fs@npm:^2.0.4":
+  version: 2.0.4
+  resolution: "@types/tar-fs@npm:2.0.4"
+  dependencies:
+    "@types/node": "npm:*"
+    "@types/tar-stream": "npm:*"
+  checksum: 10c0/d1dd6944d0905debaabe5787af7f3aeb98f13a928d688d00fb7de0411040f8556c297d388abdd046f6b0646a374b53c198ade0484060b63ef36ad5ac585df138
+  languageName: node
+  linkType: hard
+
+"@types/tar-stream@npm:*":
+  version: 3.1.3
+  resolution: "@types/tar-stream@npm:3.1.3"
+  dependencies:
+    "@types/node": "npm:*"
+  checksum: 10c0/64f87d209bd2edf1a7d029a922a246ef0dcfb19e623b95714e2c074195a61ed4fe4d67d0c3c6dc33239ef7d18902fcb70df7f7e85cfbd92a6bf25d087ce531fd
+  languageName: node
+  linkType: hard
+
 "abbrev@npm:^2.0.0":
   version: 2.0.0
   resolution: "abbrev@npm:2.0.0"
@@ -352,6 +373,13 @@ __metadata:
   languageName: node
   linkType: hard
 
+"b4a@npm:^1.6.4, b4a@npm:^1.6.6":
+  version: 1.6.6
+  resolution: "b4a@npm:1.6.6"
+  checksum: 10c0/56f30277666cb511a15829e38d369b114df7dc8cec4cedc09cc5d685bc0f27cb63c7bcfb58e09a19a1b3c4f2541069ab078b5328542e85d74a39620327709a38
+  languageName: node
+  linkType: hard
+
 "balanced-match@npm:^1.0.0":
   version: 1.0.2
   resolution: "balanced-match@npm:1.0.2"
@@ -359,6 +387,50 @@ __metadata:
   languageName: node
   linkType: hard
 
+"bare-events@npm:^2.0.0, bare-events@npm:^2.2.0":
+  version: 2.4.2
+  resolution: "bare-events@npm:2.4.2"
+  checksum: 10c0/09fa923061f31f815e83504e2ed4a8ba87732a01db40a7fae703dbb7eef7f05d99264b5e186074cbe9698213990d1af564c62cca07a5ff88baea8099ad9a6303
+  languageName: node
+  linkType: hard
+
+"bare-fs@npm:^2.1.1":
+  version: 2.3.5
+  resolution: "bare-fs@npm:2.3.5"
+  dependencies:
+    bare-events: "npm:^2.0.0"
+    bare-path: "npm:^2.0.0"
+    bare-stream: "npm:^2.0.0"
+  checksum: 10c0/ff18cc9be7c557c38e0342681ba3672ae4b01e5696b567d4035e5995255dc6bc7d4df88ed210fa4d3eb940eb29512e924ebb42814c87fc59a2bee8cf83b7c2f9
+  languageName: node
+  linkType: hard
+
+"bare-os@npm:^2.1.0":
+  version: 2.4.4
+  resolution: "bare-os@npm:2.4.4"
+  checksum: 10c0/e7d1a7b2100c05da8d25b60d0d48cf850c6f57064577a3f2f51cf18d417fbcfd6967ed2d8314320914ed69e0f2ebcf54eb1b36092dd172d8e8f969cf8cccf041
+  languageName: node
+  linkType: hard
+
+"bare-path@npm:^2.0.0, bare-path@npm:^2.1.0":
+  version: 2.1.3
+  resolution: "bare-path@npm:2.1.3"
+  dependencies:
+    bare-os: "npm:^2.1.0"
+  checksum: 10c0/35587e177fc8fa5b13fb90bac8779b5ce49c99016d221ddaefe2232d02bd4295d79b941e14ae19fda75ec42a6fe5fb66c07d83ae7ec11462178e66b7be65ca74
+  languageName: node
+  linkType: hard
+
+"bare-stream@npm:^2.0.0":
+  version: 2.3.0
+  resolution: "bare-stream@npm:2.3.0"
+  dependencies:
+    b4a: "npm:^1.6.6"
+    streamx: "npm:^2.20.0"
+  checksum: 10c0/374a517542e6a0c3c07f3a1d567db612685e66708f79781112aa0e81c1f117ec561cc1ff3926144f15a2200316a77030c95dcc13a1b96d5303f0748798b764cf
+  languageName: node
+  linkType: hard
+
 "base64-js@npm:^1.3.1":
   version: 1.5.1
   resolution: "base64-js@npm:1.5.1"
@@ -666,6 +738,13 @@ __metadata:
   languageName: node
   linkType: hard
 
+"fast-fifo@npm:^1.2.0, fast-fifo@npm:^1.3.2":
+  version: 1.3.2
+  resolution: "fast-fifo@npm:1.3.2"
+  checksum: 10c0/d53f6f786875e8b0529f784b59b4b05d4b5c31c651710496440006a398389a579c8dbcd2081311478b5bf77f4b0b21de69109c5a4eabea9d8e8783d1eb864e4c
+  languageName: node
+  linkType: hard
+
 "foreground-child@npm:^3.1.0":
   version: 3.1.1
   resolution: "foreground-child@npm:3.1.1"
@@ -1133,6 +1212,13 @@ __metadata:
   languageName: node
   linkType: hard
 
+"queue-tick@npm:^1.0.1":
+  version: 1.0.1
+  resolution: "queue-tick@npm:1.0.1"
+  checksum: 10c0/0db998e2c9b15215317dbcf801e9b23e6bcde4044e115155dae34f8e7454b9a783f737c9a725528d677b7a66c775eb7a955cf144fe0b87f62b575ce5bfd515a9
+  languageName: node
+  linkType: hard
+
 "readable-stream@npm:^3.1.1, readable-stream@npm:^3.4.0, readable-stream@npm:^3.5.0":
   version: 3.6.2
   resolution: "readable-stream@npm:3.6.2"
@@ -1274,6 +1360,21 @@ __metadata:
   languageName: node
   linkType: hard
 
+"streamx@npm:^2.15.0, streamx@npm:^2.20.0":
+  version: 2.20.1
+  resolution: "streamx@npm:2.20.1"
+  dependencies:
+    bare-events: "npm:^2.2.0"
+    fast-fifo: "npm:^1.3.2"
+    queue-tick: "npm:^1.0.1"
+    text-decoder: "npm:^1.1.0"
+  dependenciesMeta:
+    bare-events:
+      optional: true
+  checksum: 10c0/34ffa2ee9465d70e18c7e2ba70189720c166d150ab83eb7700304620fa23ff42a69cb37d712ea4b5fc6234d8e74346a88bb4baceb873c6b05e52ac420f8abb4d
+  languageName: node
+  linkType: hard
+
 "string-width-cjs@npm:string-width@^4.2.0, string-width@npm:^4.1.0":
   version: 4.2.3
   resolution: "string-width@npm:4.2.3"
@@ -1323,6 +1424,23 @@ __metadata:
   languageName: node
   linkType: hard
 
+"tar-fs@npm:^3.0.6":
+  version: 3.0.6
+  resolution: "tar-fs@npm:3.0.6"
+  dependencies:
+    bare-fs: "npm:^2.1.1"
+    bare-path: "npm:^2.1.0"
+    pump: "npm:^3.0.0"
+    tar-stream: "npm:^3.1.5"
+  dependenciesMeta:
+    bare-fs:
+      optional: true
+    bare-path:
+      optional: true
+  checksum: 10c0/207b7c0f193495668bd9dbad09a0108ce4ffcfec5bce2133f90988cdda5c81fad83c99f963d01e47b565196594f7a17dbd063ae55b97b36268fcc843975278ee
+  languageName: node
+  linkType: hard
+
 "tar-fs@npm:~2.0.1":
   version: 2.0.1
   resolution: "tar-fs@npm:2.0.1"
@@ -1348,6 +1466,17 @@ __metadata:
   languageName: node
   linkType: hard
 
+"tar-stream@npm:^3.1.5":
+  version: 3.1.7
+  resolution: "tar-stream@npm:3.1.7"
+  dependencies:
+    b4a: "npm:^1.6.4"
+    fast-fifo: "npm:^1.2.0"
+    streamx: "npm:^2.15.0"
+  checksum: 10c0/a09199d21f8714bd729993ac49b6c8efcb808b544b89f23378ad6ffff6d1cb540878614ba9d4cfec11a64ef39e1a6f009a5398371491eb1fda606ffc7f70f718
+  languageName: node
+  linkType: hard
+
 "tar@npm:^6.1.11, tar@npm:^6.1.2":
   version: 6.2.1
   resolution: "tar@npm:6.2.1"
@@ -1362,6 +1491,15 @@ __metadata:
   languageName: node
   linkType: hard
 
+"text-decoder@npm:^1.1.0":
+  version: 1.2.0
+  resolution: "text-decoder@npm:1.2.0"
+  dependencies:
+    b4a: "npm:^1.6.4"
+  checksum: 10c0/398171bef376e06864cd6ba24e0787cc626bebc84a1bbda758d06a6e9b729cc8613f7923dd0d294abd88e8bb5cd7261aad5fda7911fb87253fe71b2b5ac6e507
+  languageName: node
+  linkType: hard
+
 "tsx@npm:^4.7.1":
   version: 4.7.1
   resolution: "tsx@npm:4.7.1"