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

Triggers on push to 4.x (the production line shipped to the
Charlie/monica stack). Upstream main tracks monica v5 which isn't
deployed.

Uses scripts/docker/Dockerfile (the apache variant from monicahq/docker).
Pushes 'latest' and an immutable UTC timestamp tag to
gitea.alexandru.macocian.me/amacocian/monica.

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

.gitea/workflows/build.yaml

@@ -0,0 +1,18 @@
+name: Build image
+
+# 4.x is the production line we ship to the Charlie/monica stack.
+# `main` upstream tracks monica v5 which isn't deployed; pushes there
+# are intentionally not built.
+
+on:
+  push:
+    branches: ["4.x"]
+  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: scripts/docker/Dockerfile
+    secrets: inherit