actions: Added dockerimage-gnashplugin.yml.

Added dockerimage-gnashplugin.yml to build gnashplugin-enabled PaleMoon on demand.

.github/workflows/dockerimage-gnashplugin.yml

@@ -0,0 +1,28 @@
+name: _Manually_ build Docker image for Pale Moon browser with Gnash plugin
+
+on:
+  repository_dispatch:
+    types: ['build-with-gnashplugin']
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Build the Docker image
+        run: docker build . --file Dockerfile.gnashplugin --tag vasilev/palemoon
+      - name: Log into registry
+        run: | 
+          echo "${{ secrets.DOCKERHUB_PASSWORD }}" | docker login -u ${{ secrets.DOCKERHUB_USERNAME }} --password-stdin
+      - name: Push image
+        run: |
+          # Get the version from client payload
+          VERSION=${{ github.event.client_payload.version }}
+          # Use 'latest' if provided value is empty
+          : ${VERSION:='latest'}
+          TAGGED_IMAGENAME="vasilev/palemoon:$VERSION-gnashplugin-$(date -I)"
+          echo TAGGED_IMAGENAME=$TAGGED_IMAGENAME
+          docker tag vasilev/palemoon $TAGGED_IMAGENAME
+          docker push $TAGGED_IMAGENAME