Combine Docker GCR presubmits and also push main to gcr

.github/workflows/docker-gcr.yml

-name: Upload requested containers to gcr.io
-
-on:
-  pull_request:
-    types: [labeled]
-    paths:
-      - '.github/workflows/docker-gcr.yml'
-      - 'tf_sig_build_dockerfiles/**'
-      - '!tf_sig_build_dockerfiles/README.md'
-
-jobs:
-  docker:
-    if: ${{ github.event.label.name == 'build and push to gcr.io for staging' }}
-    runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        python-version: [python3.7, python3.8, python3.9]
-    steps:
-      -
-        name: Checkout
-        uses: actions/checkout@v2
-      -
-        name: Clear the build-and-push label
-        uses: actions-ecosystem/action-remove-labels@v1
-        with:
-          labels: build and push to gcr.io for staging
-      -
-        name: Set up QEMU
-        uses: docker/setup-qemu-action@v1
-      -
-        name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v1
-      -
-        name: Login to GCR
-        uses: docker/login-action@v1
-        with:
-          registry: gcr.io
-          username: _json_key
-          password: ${{ secrets.GCP_CREDS }}
-      -
-        name: Build and push
-        id: docker_build
-        uses: docker/build-push-action@v2
-        with:
-          push: true
-          context: ./tf_sig_build_dockerfiles
-          target: devel
-          build-args: |
-            PYTHON_VERSION=${{ matrix.python-version }}
-          tags: |
-            gcr.io/tensorflow-sigs/build:${{ github.event.number }}-${{ matrix.python-version }}
-          cache-from: |
-            type=registry,ref=tensorflow/build:latest-${{ matrix.python-version }}
-            type=registry,ref=gcr.io/tensorflow-sigs/build:${{ github.event.number }}-${{ matrix.python-version }}
-          cache-to: type=inline
-      -
-        name: Image digest
-        run: echo ${{ steps.docker_build.outputs.digest }}
-      -
-        name: Add a comment with the pushed containers
-        uses: mshick/add-pr-comment@v1
-        with:
-          repo-token: ${{ secrets.GITHUB_TOKEN }}
-          message: |
-            I pushed these containers:
-            
-            - `gcr.io/tensorflow-sigs/build:${{ github.event.number }}-python3.9`
-            - `gcr.io/tensorflow-sigs/build:${{ github.event.number }}-python3.8`
-            - `gcr.io/tensorflow-sigs/build:${{ github.event.number }}-python3.7`
-            
-            Re-apply the `build and push to gcr.io for staging` label to
-            rebuild and push again. This comment will only be posted once.

.github/workflows/docker-presubmit.yml

-# Run on pull requests.
-# Builds Docker images but doesn't push them anywhere.
-name: Docker presubmit
+name: Upload requested containers to gcr.io
 
 on:
-  # Allow manual testing on side braches
-  workflow_dispatch:
   pull_request:
+    types: [labeled]
     paths:
+      - '.github/workflows/docker-gcr.yml'
       - 'tf_sig_build_dockerfiles/**'
       - '!tf_sig_build_dockerfiles/README.md'
-    branches:
-      - master
 
 jobs:
   docker:
+    if: ${{ github.event.label.name == 'build and push to gcr.io for staging' }}
     runs-on: ubuntu-latest
     strategy:
       matrix:
         python-version: [python3.7, python3.8, python3.9]
     steps:
       -
-        name: Checkout this repository
+        name: Checkout
         uses: actions/checkout@v2
+      -
+        name: Clear the build-and-push label
+        uses: actions-ecosystem/action-remove-labels@v1
+        with:
+          labels: build and push to gcr.io for staging
       -
         name: Set up QEMU
         uses: docker/setup-qemu-action@v1
@@ -29,16 +31,42 @@ jobs:
         name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
       -
-        name: Build containers
+        name: Login to GCR
+        uses: docker/login-action@v1
+        with:
+          registry: gcr.io
+          username: _json_key
+          password: ${{ secrets.GCP_CREDS }}
+      -
+        name: Build and push
         id: docker_build
         uses: docker/build-push-action@v2
         with:
-          push: false
+          push: true
           context: ./tf_sig_build_dockerfiles
           target: devel
           build-args: |
             PYTHON_VERSION=${{ matrix.python-version }}
-          tags: tensorflow/build:latest-${{ matrix.python-version }}
-          cache-from: type=registry,ref=tensorflow/build:latest-${{ matrix.python-version }}
+          tags: |
+            gcr.io/tensorflow-sigs/build:${{ github.event.number }}-${{ matrix.python-version }}
+          cache-from: |
+            type=registry,ref=tensorflow/build:latest-${{ matrix.python-version }}
+            type=registry,ref=gcr.io/tensorflow-sigs/build:${{ github.event.number }}-${{ matrix.python-version }}
           cache-to: type=inline
+      -
+        name: Image digest
+        run: echo ${{ steps.docker_build.outputs.digest }}
+      -
+        name: Add a comment with the pushed containers
+        uses: mshick/add-pr-comment@v1
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          message: |
+            I pushed these containers:
+            
+            - `gcr.io/tensorflow-sigs/build:${{ github.event.number }}-python3.9`
+            - `gcr.io/tensorflow-sigs/build:${{ github.event.number }}-python3.8`
+            - `gcr.io/tensorflow-sigs/build:${{ github.event.number }}-python3.7`
             
+            Re-apply the `build and push to gcr.io for staging` label to
+            rebuild and push again. This comment will only be posted once.