Add cheating bootstrap CI container, with gocheckout pre-installed

.gitea/workflows/CI.yaml

@@ -3,10 +3,32 @@ name: CI
 on: [push]
 
 jobs:
-  # This step builds the `gas/ci` docker image.
-  # There's a tricky bootstrapping problem here.  Let's use Alpine and install `gocheckout` first
-  # thing.
+  # These steps build the `gas/ci` docker image.
+  # There's a tricky bootstrapping problem here.  We'll try to cheat first, and if that doesn't
+  # work, then we'll do a full bootstrapping build starting from Alpine and installing `gocheckout`
+  # first thing.
   build-docker:
+    container:
+      image: gas/ci
+      volumes:
+        - woodpecker-gocache:/go-cache-volume
+      env:
+        GOPATH: /go-cache-volume
+        GOCACHE: /go-cache-volume/build-cache
+    steps:
+      - name: checkout
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: gocheckout
+
+      - name: build CI container
+        run: docker build -t gas/ci ops/devcontainer
+
+  # Fallback: full bootstrap build.  This one is quite slow.
+  build-docker-bootstrap:
+    needs: build-docker
+    if: ${{ needs.build-docker.result == "failure" }}
+
     container:
       image: alpine:3.22
       volumes: