build: Ephemeral environments for PRs via slash command (#13189)

* First pass at ephemeral env, new Docker ci target

* Add service checks, get public IP

* Separate issue_comment and workflow_run jobs

* Refactor workflows

* Adjust comment author association

* Checkout code

* Fix image name, manage service desired task count

* Use merge commit sha

* Fix IP output, add failure comment

* Refactor comment parsing & env spinup

* Check container image publish status

* Parse AWS account ID from registry URL

* Use PR number rather than variable merge commit SHA for image tag

* Fix docker push conditional

* Push multiple tags to ECR

* Fix comment author check

* Refactor comment body check

* Provision service with active task to get correct IP

* /testenv up

* Add @mentions to PR comments, env var cleanup

.github/workflows/docker.yml

@@ -24,3 +24,19 @@ jobs:
           DOCKERHUB_TOKEN: ${{ secrets.DOCKERHUB_TOKEN }}
         run: |
           .github/workflows/docker_build_push.sh
+
+      - name: Build ephemeral env image
+        if: github.event_name == 'pull_request'
+        run: |
+          mkdir -p ./build
+          echo ${{ github.sha }} > ./build/SHA
+          echo ${{ github.event.pull_request.number }} > ./build/PR-NUM
+          docker build --target ci -t ${{ github.sha }} -t "pr-${{ github.event.pull_request.number }}" .
+          docker save ${{ github.sha }} | gzip > ./build/${{ github.sha }}.tar.gz
+
+      - name: Upload build artifacts
+        if: github.event_name == 'pull_request'
+        uses: actions/upload-artifact@v2
+        with:
+          name: build
+          path: build/