use Engage and GitHub Actions for CI

Woodpecker has a severe deficiency in that caching artifacts across CI
runs is not supported. There has been an open issue for this feature for
over 1 year [here][0]. It is also not possible to run its CI definition
file locally; creating a pull request and pushing commits is the only
way to get the CI pipeline to check your code.

In Lemmy's case, the result of these facts is that the edit-compile-test
feedback loop is drawn out to 46 minutes just for the test portion. See
[here for an example][1]. Having to wait this long is a pretty serious
reduction in current productivity and potential productivity of developers
who lose interest/patience and give up before being able to contribute a
working patch.

While the maintainers understandably ["don't want to rely on
Microsoft"][0], the repository, code review, and issue tracker all live
on GitHub already. Lemmy can reduce its feedback loop latency
substantially by making use of GitHub Actions' ability to cache
artifacts between runs. Further, by defining the vast majority of the CI
pipeline via a third party tool, Lemmy can avoid relying heavily on
*any and all* CI vendors. I argue that the tradeoff between

* adding this 72 line GitHub Actions file
* large reduction in feedback loop latency and wasted CPU recompiling
  dependencies

is massively worth it.

Additionally, this particular third party tool is also capable of being
run locally without any middleware in the way like Docker. The result is
seamless caching of intermediate artifacts when running locally, which
amounts to a reduction in CI run time from 46 minutes in Woodpecker to 3
minutes on my computer with a warm cache.

[0]: woodpecker-ci/woodpecker#758
[1]: https://woodpecker.join-lemmy.org/LemmyNet/lemmy/pipeline/72/1
[2]: https://matrix.to/#/!tUnhsBRCsiePXfsIGe:matrix.org/$kV3XDo15eRPPb3xiq3gRrYr5Iix9fTGywTC53cMDRR0?via=matrix.org&via=chapo.chat&via=tchncs.de

.github/workflows/ci.yml

@@ -0,0 +1,72 @@
+# This file just bootstraps caching and execution of the real CI system
+
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+env:
+  TERM: ansi
+
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Sync repository
+        uses: actions/checkout@v3
+        with:
+          submodules: true
+
+      # GitHub Actions' `ubuntu-latest` ships with a bunch of random software
+      # pre-installed, including a distribution of Rust that will subtly break
+      # things if we don't delete it first
+      - name: Remove conflicting pre-installed tooling
+        run: rm -rf "$HOME/.cargo" "$HOME/.rustup"
+
+      - name: Install Nix with `flakes` and `nix-command` experimental features
+        uses: cachix/install-nix-action@v21
+        with:
+          # Use nixos-unstable; we only need direnv and nix-direnv from here,
+          # which are unlikely to cause nondeterministic breakages
+          nix_path: nixpkgs=channel:nixos-unstable
+
+          extraPullNames: nix-community
+
+      - name: Configure `nix-direnv` to use flakes
+        run: |
+          mkdir -p "$HOME/.config/nixpkgs"
+          echo "[(self: super: { nix-direnv = super.nix-direnv.override { enableFlakes = true; }; })]" > ~/.config/nixpkgs/overlays.nix
+          echo 'source ~/.nix-profile/share/nix-direnv/direnvrc' > ~/.direnvrc
+
+      - name: Install `direnv` and `nix-direnv`
+        run: nix-env -f '<nixpkgs>' -i direnv -i nix-direnv
+
+      - name: Pop/push downloaded crate cache
+        uses: actions/cache@v3
+        with:
+          key: downloaded-crates
+          path: ~/.cargo
+
+      - name: Pop/push compiled crate cache
+        uses: actions/cache@v3
+        with:
+          key: compiled-crates
+          path: target
+
+      # Improves the signal-to-noise ratio of the logs of the next step
+      - name: Populate `/nix/store`
+        run: nix develop --command true
+
+      - name: Perform continuous integration
+        run: |
+          direnv allow
+
+          # Control the database lifecycle here since developers may want to do
+          # so manually while working locally rather than have Engage enforce it
+          docker compose up postgres -d
+          direnv exec . engage
+          docker compose stop