add QT_QPA_PLATFORM to tests

Author: gilesknap

.github/workflows/_test.yml

@@ -16,7 +16,6 @@ on:
 env:
   # https://github.com/pytest-dev/pytest/issues/2042
   PY_IGNORE_IMPORTMISMATCH: "1"
-  # allow tests to run headless
   ENV QT_QPA_PLATFORM: offscreen
 
 jobs:

Dockerfile

@@ -1,14 +1,16 @@
 # The devcontainer should use the developer target and run as root with podman
 # or docker with user namespaces.
 ARG PYTHON_VERSION=3.11
-FROM python:${PYTHON_VERSION} as developer
+FROM python:${PYTHON_VERSION} as system
 
-# Add any system dependencies for the developer/build environment here
+# Add any system dependencies for developer and runtime targets
 RUN apt-get update && apt-get install -y --no-install-recommends \
     graphviz \
     libqt5gui5 libxcb-xinerama0 \
     && rm -rf /var/lib/apt/lists/*
 
+FROM system as developer
+
 # Set up a virtual environment and put it in PATH
 RUN python -m venv /venv
 ENV PATH=/venv/bin:$PATH