Merge branch 'tox_gitlab' into 'master'

Configure GitLab CI

See merge request bde/photo21!4

.gitignore

@@ -45,3 +45,4 @@ backups/
 env/
 venv/
 db.sqlite3
+shell.nix

.gitlab-ci.yml

@@ -0,0 +1,25 @@
+stages:
+  - test
+  - quality-assurance
+
+# Debian Bullseye
+py39-django22:
+  stage: test
+  image: debian:bullseye
+  before_script:
+    - >
+        apt-get update &&
+        apt-get install --no-install-recommends -y
+        python3-django python3-django-crispy-forms
+        python3-pil python3-pip python3-setuptools tox
+  script: tox -e py39-django22
+
+linters:
+  stage: quality-assurance
+  image: debian:bullseye
+  before_script:
+    - apt-get update && apt-get install -y tox
+  script: tox -e linters
+
+  # Be nice to new contributors, but please use `tox`
+  allow_failure: true