Keep the module cache out of the build stage's chown (closes #43)

The build stage handed /src and the whole Go module cache to the
unprivileged user with chown -R, in a layer that re-ran on every source
change. On this host that step took from about 80 s to over ten minutes,
depending on load.

The module cache now sits at /go/pkg/mod and stays root's: bootstrap
fills it as root. The sources are copied with --chown. A small chown,
cached with bootstrap, hands builder the /src directory itself, the
module cache's cache/download directory (where make build saves its
lookup of this module's own version), and the telemetry files root's go
commands left in its home. Tests and the build still run as builder.

Model: opus-5-5

Dockerfile

@@ -51,14 +51,21 @@ RUN echo "gate lint, epoch ${CHECK_EPOCH}" && \
 FROM golang@sha256:56961d79ea8129efddcc0b8643fd8a5416b4e6228cfd477e3fd61deb2672c587 AS builder
 
 # We never build or run as root. Create an unprivileged user and point
-# HOME and the Go caches at its home so go build and go test can write
-# their caches when we drop to it below. $GOPATH/bin is deliberately not
-# on PATH: script/bootstrap no longer `go install`s anything (the linter
-# runs from a pinned image, never from a host install), so nothing lands
+# HOME and the build cache at its home so go build and go test can write
+# it when we drop to it below. $GOPATH/bin is deliberately not on PATH:
+# script/bootstrap no longer `go install`s anything (the linter runs
+# from a pinned image, never from a host install), so nothing lands
 # there and adding it would only widen what this image resolves.
+#
+# The module cache is kept outside that home, at the base image's
+# default /go/pkg/mod, and belongs to root: script/bootstrap fills it as
+# root. Do not move it into the home and hand it over with `chown -R`:
+# that walks every file in it, which took from about 80 s to over ten
+# minutes on a shared host, depending on load.
 RUN adduser -D -u 1000 builder
 ENV HOME=/home/builder
 ENV GOPATH=/home/builder/go
+ENV GOMODCACHE=/go/pkg/mod
 ENV GOCACHE=/home/builder/.cache/go-build
 
 WORKDIR /src
@@ -83,11 +90,22 @@ COPY script/ script/
 COPY go.mod go.sum ./
 RUN script/bootstrap
 
-COPY . .
+# Hand builder only what it writes to, without walking the module cache.
+# This layer stays cached with bootstrap.
+# - /src itself: make build writes the binary into it, and git refuses
+#   a repository whose top directory belongs to another user.
+# - the module cache's cache/download directory itself, not what is in
+#   it: Go only reads the downloaded modules, but make build saves its
+#   lookup of this module's own version from git there, in a new
+#   directory named after the module path.
+# - builder's home: the go commands bootstrap ran as root left Go's
+#   telemetry files there, a few small files.
+RUN chown builder:builder /src /go/pkg/mod/cache/download && \
+    chown -R builder:builder /home/builder
 
-# Hand the sources and caches to the unprivileged user, then drop root
-# before running any checks or builds.
-RUN chown -R builder:builder /src /home/builder
+# The sources are handed to builder as they are copied, so no layer has
+# to walk them. Then drop root before running any checks or builds.
+COPY --chown=builder:builder . .
 USER builder
 
 # Fail the build unless the branch is green. Runs as non-root so the