Dockerfile: reduce trips to github.com; take advantage of WORKDIR

No functionality changes expected.

docker/Dockerfile

@@ -14,18 +14,18 @@ RUN apt update
 RUN apt install --yes graphviz ${EXTRA_DEBIAN_PACKAGES}
 # Store a list of all installed packages. Aeromancy will include this as
 # metadata for each task.
-RUN dpkg-query -f '${binary:Package}=${Version}\n' -W > /base/packages_list.txt
+RUN dpkg-query -f '${binary:Package}=${Version}\n' -W > ./packages_list.txt
 
 # Set up Python environment with pdm.
 RUN pip3 install pdm
 
-# pdm needs to see README.md and src/ (can be empty) to validate the project.
-COPY --from=project pyproject.toml pdm.lock README.md /base/
-RUN mkdir /base/src
-
 # To access private repos on GitHub, we need to add it as a known host.
 RUN mkdir -p -m 0700 ~/.ssh && ssh-keyscan github.com >> ~/.ssh/known_hosts
 
+# pdm needs to see README.md and src/ (can be empty) to validate the project.
+COPY --from=project pyproject.toml pdm.lock README.md ./
+RUN mkdir ./src
+
 # --mount=type=ssh allows us to use forwarded SSH authentication.
 # --no-self since we haven't copied our project-specific code here yet (we do
 # that last since it's the most variable step and should help keep image build
@@ -35,4 +35,4 @@ RUN --mount=type=ssh pdm install --prod --no-self
 # Include Python code.
 # Note that `project` must be mapped as a build context:
 # https://docs.docker.com/engine/reference/commandline/buildx_build/#build-context
-COPY --from=project ./src/ /base/src/
+COPY --from=project ./src/ ./src/