fixup: Extend module path support to instrumentation

tests/antithesis/Dockerfile.builder-instrumented

@@ -9,14 +9,19 @@ FROM docker.io/antithesishq/go-instrumentor AS instrumentor
 FROM golang:$GO_VERSION-bullseye
 
 WORKDIR /build
-# Copy and download dependencies using go mod
-COPY go.mod .
-COPY go.sum .
-RUN go mod download
 
 # Copy the code into the container
 COPY . .
 
+# Providing a non-empty MODULE_PATH supports the case where the target module is
+# not at the root of the repo but its go.mod references other modules in the repo
+# (including the root).
+ARG MODULE_PATH=.
+
+# Download dependencies for the specified go module if a path was provided,
+# otherwise download dependencies for the current directory.
+RUN cd $MODULE_PATH && go mod download
+
 # Ensure pre-existing builds are not available for inclusion in the final image
 RUN [ -d ./build ] && rm -rf ./build/* || true
 
@@ -38,7 +43,7 @@ RUN cp -r .git /opt/tmp/
 RUN /opt/antithesis/bin/goinstrumentor \
     -stderrthreshold=INFO \
     -antithesis /opt/antithesis/instrumentation \
-    . \
+    $MODULE_PATH \
     /instrumented
 
 WORKDIR /instrumented/customer

tests/antithesis/Dockerfile.builder-uninstrumented

@@ -9,16 +9,14 @@ WORKDIR /build
 # Copy the code into the container
 COPY . .
 
-# Download dependencies of the main go module
-RUN go mod download
-
 # Providing a non-empty MODULE_PATH supports the case where the target module is
 # not at the root of the repo but its go.mod references other modules in the repo
 # (including the root).
-ARG MODULE_PATH
+ARG MODULE_PATH=.
 
-# Download dependencies for the specified go module if a path was provided
-RUN [ -n $MODULE_PATH ] && (cd $MODULE_PATH && go mod download) || true
+# Download dependencies for the specified go module if a path was provided,
+# otherwise download dependencies for the current directory.
+RUN cd $MODULE_PATH && go mod download
 
 # Ensure pre-existing builds are not available for inclusion in the final image
 RUN [ -d ./build ] && rm -rf ./build/* || true