add woodpecker ci

so pipelines don't get killed by OOM.

.woodpecker/flake-check.yaml

@@ -0,0 +1,8 @@
+when:
+  - event: pull_request
+
+steps:
+  - name: "Run nix flake check"
+    image: docker.nix-community.org/nixpkgs/nix-flakes
+    commands:
+      - nix flake check

config/hosts/woodpecker/woodpecker-server/woodpecker-server.nix

@@ -22,6 +22,7 @@
       WOODPECKER_DATABASE_DATASOURCE = "postgresql://woodpecker-server@/woodpecker-server?host=/run/postgresql";
       WOODPECKER_FORGEJO = "true";
       WOODPECKER_FORGEJO_URL = "https://git.hamburg.ccc.de";
+      WOODPECKER_LIMIT_MEM = "6442450944"; # 6GB
       # Set via enviornmentFile:
       # WOODPECKER_FORGEJO_CLIENT
       # WOODPECKER_FORGEJO_SECRET