Add builder workflow

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

.gitea/workflows/builder.yaml

@@ -0,0 +1,48 @@
+name: Build & Push Builder Image
+
+on:
+  workflow_call:
+    inputs:
+      image:
+        description: "Full image name to build and push"
+        required: true
+        type: string
+
+      username:
+        required: true
+        type: string
+
+    secrets:
+      password:
+        required: true
+
+    outputs:
+      tag:
+        description: "The built image reference"
+        value: ${{ jobs.builder.outputs.tag }}
+
+jobs:
+  builder:
+    runs-on: nix
+    outputs:
+      tag: ${{ steps.build.outputs.tag }}
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v3
+
+      - name: Log In to Registry
+        run: buildah login --username "${{ inputs.username }}" --password "${{ secrets.password }}" "$(echo "${{ inputs.image }}" | cut -d/ -f1)"
+
+      - name: Pull Builder Cache
+        run: buildah pull ${{ inputs.image }} || true
+
+      - name: Build Builder Image
+        id: build
+        run: |
+          buildah build --layers -f .gitea/Containerfile -t ${{ inputs.image }} .
+          digest=$(skopeo inspect containers-storage:${{ inputs.image }} | jq -r '.Digest')
+          ref="${{ inputs.image }}@${digest}"
+          echo "tag=$ref" >> $GITEA_OUTPUT
+
+      - name: Push Builder Image
+        run: skopeo copy containers-storage:${{ inputs.image }} docker://${{ inputs.image }}