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/cibuild

@@ -6,7 +6,8 @@
 # the lower limit of a plain `docker build .`.
 # A cached build checks nothing: a new CHECK_EPOCH on every run makes the
 # Dockerfile's check steps run again on an unchanged tree, while its base
-# images and module downloads stay cached.
+# images, module downloads and the Go build cache that make test and make
+# build use stay cached.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"