feat(ci): set up automated deploys to fly.io

.github/workflows/deploy.yml

@@ -0,0 +1,47 @@
+name: Deploy to fly.io
+on:
+  workflow_dispatch:
+  push:
+    tags:
+      - 'v[0-9]+.[0-9]+.[0-9]+'
+
+env:
+  FLY_API_TOKEN: ${{ secrets.FLY_API_TOKEN }}
+
+jobs:
+  deploy:
+    name: Deploy app
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@755da8c3cf115ac066823e79a1e1788f8940201b # v3
+
+    - name: Set up flyctl
+      uses: superfly/flyctl-actions/setup-flyctl@master
+
+    - name: Install Nix
+      uses: cachix/install-nix-action@daddc62a2e67d1decb56e028c9fa68344b9b7c2a # tag=v18
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+        extra_nix_config:
+          access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+
+    - name: Set up Cachix (msfjarvis)
+      uses: cachix/cachix-action@6a9a34cdd93d0ae4b4b59fd678660efb08109f2f # tag=v12
+      with:
+        name: msfjarvis
+        authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
+        cachixArgs: -c9 --omit-deriver
+
+    - name: Build and load container image
+      shell: bash
+      run: |
+        nix build .#container
+        docker load < result
+
+    - name: Publish image and deploy
+      shell: bash
+      run: |
+        fly docker auth
+        docker push registry.fly.io/linkleaner:latest-x86_64-linux
+        fly deploy