Keep Go's build cache between builds (closes #124)

The Dockerfile runs make test and make build with Go's build cache in a
BuildKit cache mount, so a build compiles only what changed since the
last one instead of the standard library and every dependency from
nothing. The mount has an id of its own, so builds of other repositories,
compiled against other C headers, do not share it. script/test passes
-count=1, so no test result is taken from the cache.

Model: opus-5-5

script/test

@@ -9,7 +9,9 @@ main() {
     # CGO is required (Makefile exports this too)
     export CGO_ENABLED=1
     go vet ./...
-    go test ./... || go test -v ./...
+    # -count=1: run every test, never take a result from Go's test cache,
+    # which the Dockerfile keeps between builds
+    go test -count=1 ./... || go test -count=1 -v ./...
 }
 
 main "$@"