chore(ci): add ACT guards to skip DockerHub steps in local act runs

act sets ACT=true automatically. Guards added:
- docker login: if: ${{ !env.ACT }}
- build outputs: type=docker (local load) when ACT, push-by-digest when CI
- digest export/upload: if: ${{ !env.ACT }}
- merge job: if: ${{ !env.ACT }}
- release-helm job (docker.yml): if: ${{ !env.ACT }}
- version-bump git push (docker.yml): wrapped in [ -z "$ACT" ] shell guard

Run locally with:
  ./bin/act -j build -W .github/workflows/docker.yml \
    -P ubuntu-latest=catthehacker/ubuntu:act-latest

.github/workflows/docker-dev.yml

@@ -87,6 +87,7 @@ jobs:
       - uses: docker/setup-buildx-action@v3
 
       - uses: docker/login-action@v3
+        if: ${{ !env.ACT }}
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
@@ -97,18 +98,20 @@ jobs:
         with:
           context: .
           platforms: ${{ matrix.platform }}
-          outputs: type=image,name=mauriceboe/trek,push-by-digest=true,name-canonical=true,push=true
+          outputs: ${{ env.ACT == 'true' && 'type=docker' || 'type=image,name=mauriceboe/trek,push-by-digest=true,name-canonical=true,push=true' }}
           no-cache: true
           build-args: |
             APP_VERSION=${{ needs.version-bump.outputs.version }}
 
       - name: Export digest
+        if: ${{ !env.ACT }}
         run: |
           mkdir -p /tmp/digests
           digest="${{ steps.build.outputs.digest }}"
           touch "/tmp/digests/${digest#sha256:}"
 
       - name: Upload digest artifact
+        if: ${{ !env.ACT }}
         uses: actions/upload-artifact@v4
         with:
           name: digests-${{ env.PLATFORM_PAIR }}
@@ -117,6 +120,7 @@ jobs:
           retention-days: 1
 
   merge:
+    if: ${{ !env.ACT }}
     runs-on: ubuntu-latest
     needs: [version-bump, build]
     steps: