ci: add initial workflows

Signed-off-by: Christoph Heiss <christoph@c8h4.io>

.forgejo/workflows/flake.yaml

@@ -0,0 +1,21 @@
+---
+name: flake
+
+on:
+  push:
+
+env:
+  NIX_CONFIG: 'experimental-features = nix-command flakes'
+
+jobs:
+  build:
+    runs-on: nixos-unstable
+    steps:
+      - name: prepare nix container
+        run: nix-env -iA nixpkgs.nodejs
+
+      - name: checkout repository
+        uses: actions/checkout@v4
+
+      - name: run flake check
+        run: nix flake check -L

.forgejo/workflows/nightly.yaml

@@ -0,0 +1,37 @@
+---
+name: nightly build
+
+on:
+  push:
+    branches:
+      - main
+
+env:
+  NIX_CONFIG: 'experimental-features = nix-command flakes'
+
+jobs:
+  build:
+    runs-on: nixos-unstable
+    steps:
+      - name: prepare nix container
+        run: nix-env -iA nixpkgs.nodejs
+
+      - name: checkout repository
+        uses: actions/checkout@v4
+
+      - name: run flake check
+        run: nix flake check -L
+
+      - name: build nightly binary
+        run: nix build .#tiny-raytracer
+
+      - name: upload binary
+        # v4 has a bug: https://github.com/go-gitea/gitea/issues/31354
+        # use v3 until it is fixed
+        uses: forgejo/upload-artifact@v3
+        with:
+          name: tiny-raytracer-nightly-${{ env.GITHUB_SHA }}
+          path: result/bin/tiny-raytracer
+          retention-days: 14
+          if-no-files-found: error
+          overwrite: true