Merge GitHub Actions to lint, test and build and push Docker image.

.github/workflows/docker_build_push.yml

@@ -1,12 +1,27 @@
 # Taken from: https://docs.github.com/en/free-pro-team@latest/actions/guides/building-and-testing-python
 # and: https://docs.github.com/en/free-pro-team@latest/actions/guides/publishing-docker-images
 # and adapted. More info on workflow syntax: https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-syntax-for-github-actions
-name: Build and push Docker image upon release
+name: Lint the code, run tests, build and push Docker image
 
 on:
-  # Build and push Docker images *only* for releases.
+  push:
+    # Match all branches *without* a / in the name.
+    # This will exclude "sub" branches, such as "feature branches",
+    # named something like: "feature/my-feature". Those branches will
+    # likely contain "work in progress" code, so we will run the tests
+    # on the local machine and avoid using up GitHub Actions credits
+    # (if applicable, such as in `private` repositories).
+    # Filter syntax info: https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-syntax-for-github-actions#filter-pattern-cheat-sheet
+    branches:
+      - '*'
+  pull_request:
+    # Since pull requests come from "third parties", we will automatically
+    # test everything coming from the outside world, for good measure.
+    # The `branches` tag could also be omitted, yielding the same effect here.
+    branches:
+      - '**'
   release:
-    types: [published] # , created, edited
+    types: [published, created, edited]
 
 jobs:
   # Run code tests before building the image, to ensure it is good to go
@@ -23,30 +38,36 @@ jobs:
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
+
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
         pip install flake8
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+
     - name: Lint the code with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names
         flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
         # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
         flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
+
     - name: Test with unittest
       run: |
         python -m unittest discover
 
   # Build and push the Docker image
   push_to_registry:
     name: Build and push Docker image to Docker Hub
+    # ...but only when a `release` is `published`
+    if: github.event_name == 'release' && github.event.action == 'published'
     runs-on: ubuntu-latest
     steps:
       - name: Check out the repo
         uses: actions/checkout@v2
         # if: ${{ github.ref == 'ref/head/master' }} # Info: https://docs.github.com/en/free-pro-team@latest/actions/reference/context-and-expression-syntax-for-github-actions#about-contexts-and-expressions
-      - name: Push to Docker Hub
+
+      - name: Build and push image to the Docker Hub
         uses: docker/build-push-action@v1 # Info: https://github.com/docker/build-push-action/tree/releases/v1#tags
         # if: ${{ github.ref == 'ref/head/master' }}
         with: