Container image for migrator does not exist

Problem
-------

Deployments of the Unity Auth Service will need to migrate their
database.  A convenient way of delivering this is a container image
that performs the necessary migrations.  The Unity Auth Service does
not have a container image for migrating the database.

Solution
--------

Add a Dockerfile and GitHub Actions workflow that builds a migrator
image.  The image will be pushed to the GitHub Container Registry.

.github/workflows/migrator_image.yml

@@ -0,0 +1,60 @@
+name: Create and publish a Docker image
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+      # TODO: Remove this before merge.
+      - migrator
+    tags:
+      - v*
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push-image:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          submodules: true
+
+      - name: Log in to the Container registry
+        uses: docker/login-action@343f7c4344506bcbf9b4de18042ae17996df046d
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@9dc751fe249ad99385a2583ee0d084c400eee04e
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}-migrator
+          tags: |
+            type=schedule
+            type=ref,event=branch
+            type=ref,event=tag
+            type=ref,event=pr
+            type=sha
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@4a13e500e55cf31b7a5d59a38ab2040ab0f42f56
+        with:
+          context: .
+          file: MigratorDockerfile
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}

.gitignore

@@ -1,3 +1,4 @@
+*~
 /.gradle/
 /.idea/
 /.micronaut/

MigratorDockerfile

@@ -0,0 +1,3 @@
+FROM flyway/flyway:10-alpine
+
+COPY src/main/resources/db/migration /flyway/sql