ci: Move from Drone to GitHub Actions for CI

Move the building and publishing of the OCI images in this repository
from Drone to GitHub Actions. We are turning down Drone.

.github/workflows/build.yaml

@@ -8,9 +8,55 @@
 name: Build FPM Images
 on:
   workflow_dispatch:
+    inputs:
+      version:
+        description: The version to use in the image label
+        required: true
+        type: string
+  push:
+    tags: [ '*' ]
+
 jobs:
-  dummy:
+  build:
+    strategy:
+      matrix:
+        include:
+          - directory: centos/8
+            image: fpm-centos:8
+          - directory: debian/12
+            image: fpm-debian:12
     runs-on: ubuntu-latest
     steps:
-      - name: Greeting
-        run: echo "This is a dummy build to placate GitHub."
+      - name: version
+        env:
+          VERSION: ${{ inputs.version }}
+          REF_NAME: ${{ github.ref_name }}
+        run: |
+          V="${VERSION:-${REF_NAME}}"
+          if [[ -z "$V" ]]; then
+            echo "No version or ref_name set" >&2
+            exit 1
+          fi
+          echo "V=$V" >> "$GITHUB_ENV"
+      - name: Assume AWS ECR Push Role
+        uses: aws-actions/configure-aws-credentials@010d0da01d0b5a38af31e9c3470dbfdabdecca3a # v4.0.1
+        with:
+          aws-region: us-west-2
+          role-to-assume: arn:aws:iam::146628656107:role/fpm-oci-tag-build-gha
+          role-session-name: "gha-fpm-build=${{ github.run_number }}"
+          role-duration-seconds: 900
+      - name: Log into ECR
+        uses: docker/login-action@v3
+        with:
+          registry: public.ecr.aws
+        env:
+          AWS_REGION: us-east-1
+      - name: Build FPM Image
+        uses: docker/build-push-action@v3
+        with:
+          push: true
+          context: ${{ matrix.directory }}
+          dockerfile: ${{ matrix.directory }}/Dockerfile
+          platforms: linux/amd64,linux/arm64
+          tags: |
+            public.ecr.aws/gravitational/${{ matrix.image }}-${{ env.V }}