fix(ci): docker metadata image tags

- Move metata as top step in publish workflow
- Simplify `v0.x.x` check
- Dynamically determine base branch

.github/workflows/ci.yml

@@ -91,22 +91,6 @@ jobs:
     steps:
       - uses: actions/checkout@v2.4.0
 
-      - uses: docker/setup-qemu-action@v1
-      - uses: docker/setup-buildx-action@v1
-
-      - uses: docker/login-action@v1
-        with:
-          username: qmcgaw
-          password: ${{ secrets.DOCKERHUB_PASSWORD }}
-
-      - name: Check for semver tag
-        id: semvercheck
-        run: |
-          if [[ ${{ github.ref }} =~ ^refs\/tags\/v[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
-            MATCH=true
-          fi
-          echo ::set-output name=match::$MATCH
-
       # extract metadata (tags, labels) for Docker
       # https://github.com/docker/metadata-action
       - name: Extract Docker metadata
@@ -114,16 +98,25 @@ jobs:
         uses: docker/metadata-action@v3
         with:
           flavor: |
-            latest=${{ github.ref == 'refs/heads/master' }}
+            latest=${{ github.ref == format('refs/heads/{0}', github.event.repository.default_branch) }}
           images: |
             qmcgaw/gluetun
             qmcgaw/private-internet-access
           tags: |
-            type=ref,event=branch,enable=${{ github.ref != 'refs/heads/master' }}
-            type=semver,pattern=v{{major}}.{{minor}}.{{patch}},enable=${{ startsWith(steps.semvercheck.outputs.match, 'true') }}
-            type=semver,pattern=v{{major}}.{{minor}},enable=${{ startsWith(steps.semvercheck.outputs.match, 'true') }}
-            type=semver,pattern=v{{major}},enable=${{ startsWith(steps.semvercheck.outputs.match, 'true_nonzero') }}
-            type=raw,value=latest,enable=${{ !startsWith(steps.semvercheck.outputs.match, 'true') }}
+            type=ref,event=branch,enable=${{ github.ref != format('refs/heads/{0}', github.event.repository.default_branch) }}
+            type=ref,event=pr
+            type=semver,pattern=v{{major}}.{{minor}}.{{patch}}
+            type=semver,pattern=v{{major}}.{{minor}}
+            type=semver,pattern=v{{major}},enable=${{ !startsWith(github.ref, 'refs/tags/v0.') }}
+            type=raw,value=latest,enable=${{ github.ref == format('refs/heads/{0}', github.event.repository.default_branch) }}
+
+      - uses: docker/setup-qemu-action@v1
+      - uses: docker/setup-buildx-action@v1
+
+      - uses: docker/login-action@v1
+        with:
+          username: qmcgaw
+          password: ${{ secrets.DOCKERHUB_PASSWORD }}
 
       - name: Short commit
         id: shortcommit