docker: add support for latest setuptools

Configure setuptools to use distutils provided by the standard Python
library instead of the vendored one in setuptools, so that editable
installations are stored in the right directory.

Dockerfile

@@ -10,6 +10,11 @@ FROM docker.io/library/ubuntu:20.04
 # Use default answers in installation commands
 ENV DEBIAN_FRONTEND=noninteractive
 
+# Use distutils provided by the standard Python library instead of the vendored one in
+# setuptools, so that editable installations are stored in the right directory.
+# See https://github.com/pypa/setuptools/issues/3301
+ENV SETUPTOOLS_USE_DISTUTILS=stdlib
+
 # Prepare list of Python dependencies
 COPY requirements.txt /code/
 
@@ -23,7 +28,7 @@ RUN apt-get update -y && \
       python3-dev \
       python3-pip \
       vim-tiny && \
-    pip install --no-cache-dir --upgrade pip && \
+    pip install --no-cache-dir --upgrade pip setuptools && \
     pip install --no-cache-dir -r /code/requirements.txt && \
     apt-get remove -y \
       gcc \