diff --git a/Dockerfile b/Dockerfile
index d0afa31..bd16c07 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -50,15 +50,26 @@ ARG CHECK_EPOCH
 
 COPY . .
 
-RUN make test
+# 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.
+RUN --mount=type=cache,id=sneak/secret/go-build,target=/root/.cache/go-build \
+    make test
 
 # The version stamped into the binary: the VERSION build argument when one
 # 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.
+# and still yields no version fails the build. make build uses the same Go
+# build cache mount as make test.
 ARG VERSION
-RUN version="${VERSION:-$(git describe --tags --always)}"; \
+RUN --mount=type=cache,id=sneak/secret/go-build,target=/root/.cache/go-build \
+    version="${VERSION:-$(git describe --tags --always)}"; \
     if [ -e .git ] && { [ -z "$version" ] || [ "$version" = dev ] || \
         [ "$version" = unknown ]; }; then \
         echo "no version could be derived although the build context carries .git" >&2; \
diff --git a/README.md b/README.md
index 29d38f7..0f44d03 100644
--- a/README.md
+++ b/README.md
@@ -604,7 +604,8 @@ provide:
 - `script/build` — build the `secret` binary into the repo root, stamping the
   version (`VERSION` from the environment, else `git describe`) and the git
   commit
-- `script/test` — run `go vet` and the test suite (verbose rerun on failure)
+- `script/test` — run `go vet` and the test suite (verbose rerun on failure),
+  every test on every run, never a result from Go's test cache
 - `script/lint` — run `golangci-lint` in docker only: builds `Dockerfile.lint`,
   where the linter is a build step that runs on every call, also on an unchanged
   tree
@@ -624,7 +625,8 @@ provide:
 - `script/cibuild` — CI entrypoint: `docker build --ulimit memlock=-1:-1 .`
   (memguard needs mlock; the Dockerfile runs the checks), with a new
   `CHECK_EPOCH` build argument on every run so the checks run again on an
-  unchanged tree
+  unchanged tree; the `Dockerfile` keeps Go's build cache between builds, so
+  `make test` and `make build` compile only what changed
 - `script/precommit` — pre-commit checks: `go mod tidy` verification, then
   `script/check`
 - `script/install-precommit` — install the git pre-commit hook that runs
diff --git a/TODO.md b/TODO.md
index 37ea5a0..c388fa9 100644
--- a/TODO.md
+++ b/TODO.md
@@ -18,6 +18,16 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-06: `make test` in `script/cibuild` no longer compiles the standard
+  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.
 - 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
diff --git a/script/cibuild b/script/cibuild
index bb21515..feaf540 100755
--- a/script/cibuild
+++ b/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)"
diff --git a/script/test b/script/test
index 3735b43..7a48c8b 100755
--- a/script/test
+++ b/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 "$@"