ci: build + push invidious image via Charlie build-image workflow

Pushes 'latest' and an immutable UTC timestamp tag to
gitea.alexandru.macocian.me/amacocian/invidious on every push to master.

Uses docker/Dockerfile (the amd64 image; arm64 variant is unused in
the fleet for now).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitea/workflows/build.yaml

@@ -0,0 +1,14 @@
+name: Build image
+
+on:
+  push:
+    branches: [master]
+  workflow_dispatch:
+
+jobs:
+  build:
+    uses: https://${{ github.repository_owner }}:${{ secrets.CALLER_REPO_PAT }}@gitea.alexandru.macocian.me/Charlie/project-charlie/.gitea/workflows/build-image.yml@main
+    with:
+      host: melina
+      dockerfile: docker/Dockerfile
+    secrets: inherit