(core) regularly freshen grist-core:latest docker image

Summary:
 * Keeps `grist-core:latest` docker image up to date with grist-core
   source, with a daily cron job.
 * Update yarn.lock file.
 * In passing, tag current version for a versioned release.

When a push to `main` branch passes tests, the result is placed in `latest_candidate` branch. The cron job will update docker up from `latest_candidate`, placing code used for image in `latest` branch.

Test Plan: tested in a grist-core fork

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3156

.github/workflows/docker.yml

@@ -18,5 +18,5 @@ jobs:
         with:
           username: ${{ secrets.DOCKER_USERNAME }}
           password: ${{ secrets.DOCKER_PASSWORD }}
-          repository: gristlabs/grist
+          repository: ${{ github.repository }}
           tag_with_ref: true

.github/workflows/docker_latest.yml

@@ -0,0 +1,35 @@
+name: Push latest Docker image
+
+on:
+  push:
+    # Trigger if latest_candidate updates. This is automatically done by another
+    # workflow whenever tests pass on main - but events don't chain without using
+    # personal access tokens so we just use a cron job.
+    branches: [ latest_candidate ]
+  schedule:
+    # Run at 5:41 UTC daily
+    - cron:  '41 5 * * *'
+  workflow_dispatch:
+
+jobs:
+  push_to_registry:
+    name: Push latest Docker image to Docker Hub
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out the repo
+        uses: actions/checkout@v2
+        with:
+          ref: latest_candidate
+      - name: Push to Docker Hub
+        uses: docker/build-push-action@v1
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+          repository: ${{ github.repository }}
+          tags: latest
+      - name: Update latest branch
+        uses: ad-m/github-push-action@8407731efefc0d8f72af254c74276b7a90be36e1
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          branch: latest
+          force: true

.github/workflows/main.yml

@@ -43,3 +43,11 @@ jobs:
 
       - name: Run tests
         run: VERBOSE=1 DEBUG=1 MOCHA_WEBDRIVER_HEADLESS=1 yarn run test
+
+      - name: Update candidate branch
+        if: ${{ github.event_name == 'push' }}
+        uses: ad-m/github-push-action@8407731efefc0d8f72af254c74276b7a90be36e1
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          branch: latest_candidate
+          force: true