build: build and publish image nightly

Removes need to update toolchain for every action step

.forgejo/workflows/build-image.yml

@@ -0,0 +1,18 @@
+on:
+  schedule:
+    - cron: '30 1 * * *'
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: docker
+
+    steps:
+        - name: Checkout
+          uses: actions/checkoutv4
+
+        - name: Build
+          run: docker build . -t git.kemitix.net/kemitix/rust:latest
+
+        - name: Publish
+          run: docker push git.kemitix.net/kemitix/rust:latest