GitHub: add nightly schedule to Docker CD workflow

Added a daily schedule to the Docker workflow to automate builds and
pushes of the `daily-testing` image. This change introduces a new cron
job that runs every day at midnight (UTC) and triggers the workflow.

Author: nehhen

.github/workflows/docker.yml

@@ -4,7 +4,11 @@ on:
   push:
     tags:
       - 'v*'
-
+  schedule:
+    # Every day at midnight (UTC).
+    - cron: '0 0 * * *'
+  # Manual trigger through the UI for testing.
+  workflow_dispatch:
 
 defaults:
   run:
@@ -13,6 +17,11 @@ defaults:
 env:
   DOCKER_ORG: lightninglabs
   DOCKER_REPO: lightning-terminal
+  NIGHTLY_DOCKER_REPO: lightning-terminal-nightly
+  # TODO(guggero): Change the branch to 'master' once the experimental branch is
+  # merged into master.
+  NIGHTLY_BRANCH_NAME: 0-19-staging
+  NIGHTLY_TAG_NAME: experimental-daily-testing
 
 jobs:
   main:
@@ -37,6 +46,15 @@ jobs:
           echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
           echo "IMAGE_TAG=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
 
+      # The daily/nightly build (or manual trigger) will always use the
+      # experimental branch and push to a different docker repo.
+      - name: Set daily tag
+        if: ${{ github.event.schedule == '0 0 * * *' || github.event_name == 'workflow_dispatch' }}
+        run: |
+          echo "RELEASE_VERSION=${{env.NIGHTLY_BRANCH_NAME}}" >> $GITHUB_ENV
+          echo "DOCKER_REPO=${{env.NIGHTLY_DOCKER_REPO}}" >> $GITHUB_ENV
+          echo "IMAGE_TAG=${{env.NIGHTLY_TAG_NAME}}-$(date -u +%Y%m%d)" >> $GITHUB_ENV
+
       - name: Build and push default image
         id: docker_build
         uses: lightninglabs/gh-actions/build-push-action@2021.01.25.00