feat: add PR Docker image builds for x86_64 (#1810)

* Initial plan

* feat: add PR docker build workflow

Add a new workflow that builds and pushes Docker images for pull requests:
- Runs independently without waiting for tests
- Builds only for linux/amd64 platform
- Pushes only to GHCR (not Docker Hub)
- Tags images with pr-<number> format

Co-authored-by: kolaente <13721712+kolaente@users.noreply.github.com>

* feat: add SHA tag to PR Docker images

Add type=sha,format=long to docker tags for PR images.
This will tag images with both pr-<number> and sha-<commit_hash>.

Co-authored-by: kolaente <13721712+kolaente@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: kolaente <13721712+kolaente@users.noreply.github.com>

.github/workflows/pr-docker.yml

@@ -0,0 +1,47 @@
+name: PR Docker Build
+
+on:
+  pull_request:
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    permissions:
+      packages: write
+      contents: read
+    steps:
+      - name: Checkout
+        uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+      - name: Git describe
+        id: ghd
+        uses: proudust/gh-describe@v2
+      - name: Login to GHCR
+        uses: docker/login-action@5e57cd118135c172c3672efd75eb46360885c0ef # v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@e468171a9de216ec08956ac3ada2f0791b6bd435 # v3
+        with:
+          version: latest
+      - name: Docker meta
+        id: meta
+        uses: docker/metadata-action@318604b99e75e41977312d83839a89be02ca4893 # v5
+        with:
+          images: ghcr.io/go-vikunja/vikunja
+          tags: |
+            type=ref,event=pr
+            type=sha,format=long
+      - name: Build and push PR image
+        uses: docker/build-push-action@263435318d21b8e681c14492fe198d362a7d2c83 # v6
+        with:
+          context: .
+          platforms: linux/amd64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          build-args: |
+            RELEASE_VERSION=${{ steps.ghd.outputs.describe }}