Keep Go's build cache in check builds: re-vendor sneak/prompts e3f79c4 (closes #129)

The vendored files are copies from sneak/prompts e3f79c4, with this
repository's own entries kept. The script/ builds pass a new CACHEBUST
value instead of --no-cache, which emptied Go's build cache mount, so
every make test compiled the standard library and every dependency with
-race. Every Dockerfile stage declares ARG CACHEBUST first, so every RUN
step still reruns on an unchanged tree. The steps that compile mount
Go's build cache, and both go test runs keep -count=1. The lint and test
builds write no image. script/bootstrap, script/fmt and script/fmt-check
are the canonical files plus this repository's Go steps.

Model: opus-5-5

Dockerfile

@@ -1,33 +1,40 @@
+# Every stage declares ARG CACHEBUST first: script/ passes a new value
+# on every build, which reruns every RUN step after it. The steps that
+# compile mount Go's build cache, which survives from build to build.
+
 # Lint phase. The linter is invoked directly rather than through `make
 # lint` or `script/lint`, which are themselves a docker build.
 # golangci/golangci-lint:v2.14.0 (Debian-based), 2026-09-24
 FROM golangci/golangci-lint@sha256:ad862ba6b3798cbe0fd9fd7408d498fd74fbd2623a92406b2fd3898faf0bf98f AS lint
+ARG CACHEBUST
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-RUN go vet ./...
-RUN golangci-lint run --config .golangci.yml ./...
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    go vet ./...
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    golangci-lint run --config .golangci.yml ./...
 # The same checks on the code as a macOS build compiles it, which a Linux
 # build never compiles. Cgo is off, because compiling cgo code for macOS
 # needs Apple's SDK headers.
-RUN GOOS=darwin CGO_ENABLED=0 go vet ./...
-RUN GOOS=darwin CGO_ENABLED=0 golangci-lint run --config .golangci.yml ./...
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    GOOS=darwin CGO_ENABLED=0 go vet ./...
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    GOOS=darwin CGO_ENABLED=0 golangci-lint run --config .golangci.yml ./...
 
 # Test phase. -race needs cgo and so a C compiler, which the Debian Go
-# image ships and the alpine one does not.
+# image ships and the alpine one does not. -count=1 because the mounted
+# cache also holds test results, which go test would otherwise replay.
 # golang:1.24.13-trixie, 2026-02-04
 FROM golang@sha256:5835f052b784aa39f2fe9070def3568605c8bc3fcd810f10402066348b61e716 AS test
+ARG CACHEBUST
 ENV CGO_ENABLED=1
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-# Go's build cache goes in a cache mount, not the image layer, which would
-# take seconds longer to export. --no-cache, on every build in script/,
-# starts the mount empty; -count=1 keeps a build without it from taking
-# test results from there.
-RUN --mount=type=cache,id=sneak/secret/go-build-test,target=/root/.cache/go-build \
+RUN --mount=type=cache,target=/root/.cache/go-build \
     go test -count=1 -timeout 90s -race -cover ./... || \
     { echo "--- Rerunning with -v for details ---"; \
       go test -count=1 -timeout 90s -race -v ./...; exit 1; }
@@ -37,6 +44,7 @@ RUN --mount=type=cache,id=sneak/secret/go-build-test,target=/root/.cache/go-buil
 # lint and test passed.
 # golang 1.24.13-alpine, 2026-03-10
 FROM golang@sha256:8bee1901f1e530bfb4a7850aa7a479d17ae3a18beb6e09064ed54cfd245b7191 AS builder
+ARG CACHEBUST
 COPY --from=lint /src/go.sum /dev/null
 COPY --from=test /src/go.sum /dev/null
 # script/build compiles with cgo, so it needs a C compiler too.
@@ -53,7 +61,8 @@ COPY . .
 # .git present, a version that is still empty, dev or unknown fails the
 # build: git is missing or could not read the checkout.
 ARG VERSION
-RUN VERSION="${VERSION:-$(git describe --tags --always)}"; \
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    VERSION="${VERSION:-$(git describe --tags --always)}"; \
     if [ -e .git ]; then \
         case "$VERSION" in ""|dev|unknown) \
             echo "version is '$VERSION' although .git is present" >&2; \
@@ -65,6 +74,7 @@ RUN VERSION="${VERSION:-$(git describe --tags --always)}"; \
 # Runtime stage, and the last one
 # alpine 3.23, 2026-03-10
 FROM alpine@sha256:25109184c71bdad752c8312a8623239686a9a2071e8825f20acb8f2198c3f659
+ARG CACHEBUST
 
 RUN apk add --no-cache ca-certificates gnupg