diff --git a/.dockerignore b/.dockerignore
new file mode 100644
index 0000000..e29fbef
--- /dev/null
+++ b/.dockerignore
@@ -0,0 +1,7 @@
+__pycache__
+/.env
+/.envrc
+/.idea
+/.mypy_cache
+/.venv
+/*.egg-info
diff --git a/.gitignore b/.gitignore
index 08f46a4..10b58d8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,7 +1,8 @@
 __pycache__
 /.env
+/.envrc
+/.idea
+/.mypy_cache
+/.venv
 /dist
 /*.egg-info
-.envrc
-.venv
-.idea
diff --git a/Dockerfile b/Dockerfile
index aabd0fb..28cfc2f 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -4,14 +4,14 @@ FROM python:${PYTHON_VERSION}-alpine as builder
 
 COPY ./ ./
 
-RUN python3 -m pip install --upgrade pip && \
-    pip install build && \
-    python -m build --wheel
+RUN PIP_DISABLE_PIP_VERSION_CHECK=1 python3 -m pip install --upgrade pip && \
+    pip install build
+RUN python -m build --wheel
 
 FROM python:${PYTHON_VERSION}-alpine as host
 
-RUN apk add --update nodejs npm && \
-    npm install -g @mermaid-js/mermaid-cli
+RUN apk add --update nodejs npm
+RUN npm install -g @mermaid-js/mermaid-cli
 
 FROM host as runner
 
diff --git a/md2conf/mermaid.py b/md2conf/mermaid.py
index a1a626b..4a3653e 100644
--- a/md2conf/mermaid.py
+++ b/md2conf/mermaid.py
@@ -41,10 +41,11 @@ def render(source: str, output_format: Literal["png", "svg"] = "png") -> bytes:
             stderr=subprocess.PIPE,
             text=False,
         )
-        proc.communicate(input=source.encode("utf-8"))
+        stdout, stderr = proc.communicate(input=source.encode("utf-8"))
         if proc.returncode:
             raise RuntimeError(
-                f"failed to convert Mermaid diagram; exit code: {proc.returncode}"
+                f"failed to convert Mermaid diagram; exit code: {proc.returncode}, "
+                f"output:\n{stdout.decode('utf-8')}\n{stderr.decode('utf-8')}"
             )
         with open(filename, "rb") as image:
             return image.read()
diff --git a/package.sh b/package.sh
index 5208432..5eb7181 100755
--- a/package.sh
+++ b/package.sh
@@ -1,4 +1,7 @@
 set -e
+#
+# Builds a Python package and runs unit tests in Docker
+#
 
 PYTHON=python3
 
@@ -14,6 +17,6 @@ $PYTHON -m build
 for PYTHON_VERSION in 3.8 3.9 3.10 3.11 3.12
 do
     docker build -f test.dockerfile -t py-$PYTHON_VERSION-image --build-arg PYTHON_VERSION=$PYTHON_VERSION .
-    docker run -it --env-file .env --rm py-$PYTHON_VERSION-image python3 -m unittest discover tests
+    docker run -i -t --env-file .env --rm py-$PYTHON_VERSION-image python3 -m unittest discover tests
     docker rmi py-$PYTHON_VERSION-image
 done
diff --git a/test.dockerfile b/test.dockerfile
index f2014b8..36cddbf 100644
--- a/test.dockerfile
+++ b/test.dockerfile
@@ -1,6 +1,17 @@
 ARG PYTHON_VERSION=3.9
-FROM python:${PYTHON_VERSION}-alpine
-RUN PIP_DISABLE_PIP_VERSION_CHECK=1 python3 -m pip install --upgrade pip
+
+FROM python:${PYTHON_VERSION}-alpine as host-alpine
+
+RUN apk add --update nodejs npm
+RUN npm install -g @mermaid-js/mermaid-cli
+
+FROM python:${PYTHON_VERSION}-slim as host-slim
+
+RUN apt-get update && apt-get install --yes --no-install-recommends nodejs npm
+RUN npm install -g @mermaid-js/mermaid-cli
+
+FROM host-slim as runner
+
 COPY dist/*.whl dist/
 RUN python3 -m pip install `ls -1 dist/*.whl`
 COPY sample/ sample/