Fixing Docker Build and Push for Runner Image (#115)

A new image tag for the runner stopped being published on master merges from changes in #86.

This fixes that in the following ways:
- Tests the GH workflow on PRs w/o pushing the images
- Runner and Docker version are moved from Makefile to GH Actions workflow and are passed in as build args
- GHA workflow runs on PRs, and if the workflow file itself is changed (i.e. version bump) or the runner Docker source changes (excluding the Makefile since thats just for local dev)
- Images are pushed on push (i.e. a merge)

.github/workflows/build-runner.yml

@@ -1,14 +1,26 @@
 on:
+  pull_request:
+    branches:
+      - '**'
+    paths:
+      - 'runner/**'
+      - .github/workflows/build-runner.yml
   push:
     branches:
       - master
     paths:
-      - 'runner/**'
-
+      - runner/patched/*
+      - runner/Dockerfile
+      - runner/entrypoint.sh
+      - .github/workflows/build-runner.yml
+name: Runner
 jobs:
   build:
     runs-on: ubuntu-latest
-    name: Build runner
+    name: Build
+    env:
+      RUNNER_VERSION: 2.273.5
+      DOCKER_VERSION: 19.03.12
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -19,16 +31,21 @@ jobs:
         with:
           buildx-version: latest
 
-      - name: Login to GitHub Docker Registry
-        run: echo "${DOCKERHUB_PASSWORD}" | docker login -u "${DOCKERHUB_USERNAME}" --password-stdin
-        env:
-          DOCKERHUB_USERNAME: summerwind
-          DOCKERHUB_PASSWORD: ${{ secrets.DOCKER_ACCESS_TOKEN }}
-
       - name: Build Container Image
         working-directory: runner
         run: |
           docker buildx build \
+            --build-arg RUNNER_VERSION=${RUNNER_VERSION} \
+            --build-arg DOCKER_VERSION=${DOCKER_VERSION} \
             --platform linux/amd64,linux/arm64 \
-            --tag summerwind/actions-runner:latest \
-            -f Dockerfile . --push
+            --tag summerwind/actions-runner:v${RUNNER_VERSION} \
+            -f Dockerfile .
+
+      - name: Push Container Image
+        working-directory: runner
+        run: |
+          docker login -u summerwind --password-stdin <<<${{ secrets.DOCKER_ACCESS_TOKEN }}
+          docker push summerwind/actions-runner:v${RUNNER_VERSION}
+          docker tag summerwind/actions-runner:v${RUNNER_VERSION} summerwind/actions-runner:latest
+          docker push summerwind/actions-runner:latest
+        if: ${{ github.event_name == 'push' }}