BE: Build Docker cnt in GH action

.github/workflows/build-projectify-backend.yml

@@ -0,0 +1,14 @@
+name: Build docker image
+
+on:
+    push:
+    pull_request:
+jobs:
+    tests:
+        defaults:
+            run:
+                working-directory: backend
+        runs-on: ubuntu-latest
+        steps:
+        - uses: actions@checkout@v4
+        - run: docker build - < projectify-backend.Dockerfile

backend/projectify-backend.Dockerfile

@@ -0,0 +1,16 @@
+FROM nixos/nix:24.05 as builder
+
+WORKDIR /app
+COPY . .
+
+RUN echo "experimental-features = nix-command flakes" > /etc/nix/nix.conf
+
+RUN nix build .#projectify-backend
+# https://marcopolo.io/code/nix-and-small-containers/
+# https://mitchellh.com/writing/nix-with-dockerfiles
+RUN cp -a $(nix-store -qR result/) /tmp/nix-store-closure
+
+FROM scratch
+WORKDIR /app
+COPY --from=builder /tmp/nix-store-closure /nix/store
+COPY --from=builder /app/result /app