ci: Ensure that minimal job runs on older Python

Fixes: https://github.com/pimutils/vdirsyncer/issues/1077

.builds/tests-minimal.yml

@@ -3,11 +3,12 @@
 # TODO: It might make more sense to test with an older Ubuntu or Fedora version
 # here, and consider that our "oldest suppported environment".
 
-image: archlinux
+image: alpine/3.15
 packages:
   - docker
+  - docker-cli
   - docker-compose
-  - python-pip
+  - py3-pip
 sources:
   - https://github.com/pimutils/vdirsyncer
 environment:
@@ -18,16 +19,16 @@ environment:
   REQUIREMENTS: minimal
   # TODO: ETESYNC_TESTS
 tasks:
+  - docker: |
+      sudo addgroup $(whoami) docker
+      sudo service docker start
   - venv: |
-      python -m venv $HOME/venv
+      python3 -m venv $HOME/venv
       echo "export PATH=$HOME/venv/bin:$PATH" >> $HOME/.buildenv
   - setup: |
-      sudo systemctl start docker
       cd vdirsyncer
       make -e install-dev
   - test: |
       cd vdirsyncer
-      # Non-system python is used for packages:
-      export PATH=$PATH:~/.local/bin/
       make -e ci-test
       make -e ci-test-storage