🔨(dockerfile) fix development image to work with a different working dir

Because the `pip install --editable` command was run from an empty
directory, the `oidc2fer` directory that was later copied from the sources
did not show up as an importable module. This worked before changing
the working directory because the import falled back on looking in the
current directory.

Dockerfile

@@ -51,10 +51,15 @@ WORKDIR /app
 # Copy project file to list dependencies
 COPY ./src/satosa/pyproject.toml /app/
 
-# Install oidc2fer in editable mode along with development dependencies
-RUN pip install -e .[dev]
+# Create empty module directory to please pip install --editable
+# (without this, the oidc2fer module will not be importable because
+#  pip --editable scans the directory to create its "links")
+RUN mkdir -p /app/oidc2fer
 
-# Copy oidc2fer application (see .dockerignore)
+# Install oidc2fer in editable mode along with development dependencies
+RUN pip install --editable .[dev]
+
+# Copy oidc2fer sources (see .dockerignore)
 COPY ./src/satosa /app/
 
 # Switch to unprivileged user