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

Dockerfile

@@ -50,14 +50,10 @@ ARG CHECK_EPOCH
 
 COPY . .
 
-# Go's build cache is kept between builds in this cache mount, which make test
-# and make build both use, so each compiles only what changed since the last
-# build, not the standard library and every dependency from nothing.
-# script/test passes -count=1, so test results are never taken from it.
-# The mount has its own id because the default id, its path, is shared with
-# other repositories' builds, and Go's cache does not notice changes to C
-# libraries: an entry compiled there against other C headers could be reused
-# here. It does not notice a change of this image's own C headers either.
+# This cache mount keeps Go's build cache between builds for make test and
+# make build; -count=1 in script/test keeps test results out of it. Go's cache
+# does not notice C header changes, so the mount has its own id: change the id
+# when the C packages installed above change.
 RUN --mount=type=cache,id=sneak/secret/go-build,target=/root/.cache/go-build \
     make test
 
@@ -65,8 +61,7 @@ RUN --mount=type=cache,id=sneak/secret/go-build,target=/root/.cache/go-build \
 # is given, otherwise `git describe --tags --always` of the .git the build
 # context carries: the tag on a tagged commit, tag-N-gHASH on a commit after
 # one, the short commit when no tag is reachable. A context that carries .git
-# and still yields no version fails the build. make build uses the same Go
-# build cache mount as make test.
+# and still yields no version fails the build.
 ARG VERSION
 RUN --mount=type=cache,id=sneak/secret/go-build,target=/root/.cache/go-build \
     version="${VERSION:-$(git describe --tags --always)}"; \

TODO.md

@@ -22,12 +22,12 @@ https://git.eeqj.de/sneak/secret/milestone/12
   library and every dependency from nothing on every build
   (https://git.eeqj.de/sneak/secret/issues/124). The `Dockerfile` runs it and
   `make build` with Go's build cache in a BuildKit cache mount, which docker
-  keeps between builds, locally and on the Gitea runner alike, so each compiles
-  only what changed since the last build. The mount has an id of its own, so
-  other repositories' builds do not share it. `script/test` passes `-count=1`,
-  so every test runs on every build and no result comes from Go's test cache. A
-  build with an empty cache, such as the first after docker's build cache is
-  cleared, compiles everything in `make test` as before.
+  keeps between builds, so each compiles only what changed since the last build.
+  The mount has an id of its own, so other repositories' builds do not share it.
+  `script/test` passes `-count=1`, so every test runs on every build and no
+  result comes from Go's test cache. A build with an empty cache, such as the
+  first after docker's build cache is cleared, compiles everything in
+  `make test` as before.
 - 2026-10-06: The tests run quickly with the race detector on
   (https://git.eeqj.de/sneak/secret/issues/120). Most of their time went to
   deriving keys from passphrases with scrypt, which is slow on purpose. The new