🐛 Fix Docker pip --user install error in virtualenv

Set VIRTUAL_ENV environment variable in Dockerfile to fix the
error when building Android apps. The error was:
```
Can not perform a '--user' install. User site-packages are not
visible in this virtualenv
```

The Docker container creates a virtualenv at /home/user/.venv but
was only setting PATH, not VIRTUAL_ENV. This caused buildozer's
virtualenv detection logic to incorrectly add --user flag to pip.

Also restrict Docker image publishing to upstream repository only
by adding repository_owner check to SHOULD_PUBLISH condition. This
prevents forks from attempting to publish to container registries,
which would fail due to missing credentials and GHCR's lowercase
requirement for repository names.

Fixes #1977

Author: AndreMiras

.github/workflows/docker.yml

@@ -8,7 +8,7 @@ on:
 env:
   DOCKERHUB_IMAGE: kivy/buildozer
   GHCR_IMAGE: ghcr.io/${{ github.repository }}
-  SHOULD_PUBLISH: ${{ github.event_name == 'push' && (github.ref == 'refs/heads/master' || startsWith(github.ref, 'refs/tags/')) }}
+  SHOULD_PUBLISH: ${{ github.repository_owner == 'kivy' && github.event_name == 'push' && (github.ref == 'refs/heads/master' || startsWith(github.ref, 'refs/tags/')) }}
 
 jobs:
   build:

Dockerfile

@@ -79,7 +79,8 @@ COPY . ${SRC_DIR}
 COPY --chmod=755 entrypoint.sh /usr/local/bin/entrypoint.sh
 
 # installs buildozer and dependencies from a virtual environment
-ENV PATH="${HOME_DIR}/.venv/bin:${PATH}"
+ENV VIRTUAL_ENV="${HOME_DIR}/.venv"
+ENV PATH="${VIRTUAL_ENV}/bin:${PATH}"
 RUN pip install --upgrade "Cython<3.0" wheel pip ${SRC_DIR}
 
 ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]