ci: build docker image on PRs / tags (#2353)

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## Overview

This PR adds a workflow which will build the rollkit image on each PR,
in a follow up we can use this image to perform upgrade tests.

If it's on a PR, it will be tagged like `pr-1234`, if it's a tag or main
it will be tagged with the format `v1.0.0` or `main`.

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

- **Chores**
- Updated continuous integration workflows to support dynamic Docker
image tagging for test jobs.
- Added automated building and publishing of Docker images during
testing.
  - Introduced a placeholder for future upgrade tests.
- Removed the separate Docker build and publish workflow to streamline
processes.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: chatton

.github/workflows/ci_release.yml

@@ -29,6 +29,9 @@ jobs:
   test:
     uses: ./.github/workflows/test.yml
     secrets: inherit
+    with:
+      # tag with the pr in the format of pr-1234 or the tag / branch if it is not a PR.
+      image-tag: ${{ github.event.pull_request.number && format('pr-{0}', github.event.pull_request.number) || github.ref_name }}
 
   proto:
     uses: ./.github/workflows/proto.yml

.github/workflows/docker-build-publish.yml

@@ -3,8 +3,42 @@
 name: Tests / Code Coverage
 on:
   workflow_call:
+    inputs:
+      image-tag:
+        required: true
+        type: string
 
 jobs:
+  build-docker-image:
+    name: Build Docker Image
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push Docker image with PR tag
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: ghcr.io/${{ github.repository_owner }}/rollkit:${{ inputs.image-tag }}
+
+  upgrade-tests:
+    needs: build-docker-image
+    runs-on: ubuntu-latest
+    steps:
+      - run: exit 0 # TODO: add upgrade test uses the image built in the build-docker-image step
+
   build_all-apps:
     name: Build All Rollkit Binaries
     runs-on: ubuntu-latest