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.

Additionally, updated the workflow to set different environment
variables for daily builds, including `RELEASE_VERSION` and `IMAGE_TAG`.
These changes enable the automatic creation of daily testing images with
unique tags.

Inspired by:
https://github.com/lightningnetwork/lnd/blob/57a5e4912bced63ce13be06896f64d7e476616b4/.github/workflows/docker.yml#L7-L44

Author: nehhen

.github/workflows/docker.yml

@@ -4,7 +4,9 @@ on:
   push:
     tags:
       - 'v*'
-
+  schedule:
+    # Every day at midnight (UTC).
+    - cron: '0 0 * * *'
 
 defaults:
   run:
@@ -31,7 +33,15 @@ jobs:
           password: ${{ secrets.DOCKER_API_KEY }}
 
       - name: Set env
-        run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
+        run: |
+          echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
+          echo "IMAGE_TAG=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
+
+      - name: Set daily tag
+        if: github.event.schedule == '0 0 * * *'
+        run: |
+          echo "RELEASE_VERSION=master" >> $GITHUB_ENV
+          echo "IMAGE_TAG=daily-testing-$(date -u +%Y%m%d),${DOCKER_REPO}/${DOCKER_IMAGE}:daily-testing-only" >> $GITHUB_ENV
 
       - name: Build and push default image
         id: docker_build
@@ -53,5 +63,14 @@ jobs:
             checkout=${{ env.RELEASE_VERSION }}
             public_url=/lit
 
+      - name: Build and push nightly-build image
+        id: docker_build
+        uses: lightninglabs/gh-actions/[email protected]
+        with:
+          push: true
+          platforms: linux/amd64,linux/arm64
+          tags: "${{ env.DOCKER_REPO }}/${{ env.DOCKER_IMAGE }}:${{ env.IMAGE_TAG }}"
+          build-args: checkout=${{ env.RELEASE_VERSION }}
+
       - name: Image digest
         run: echo ${{ steps.docker_build.outputs.digest }} ${{ steps.docker_build2.outputs.digest }}