diff --git a/Dockerfile b/Dockerfile
index 2885086..98a2174 100644
--- a/Dockerfile
+++ b/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
diff --git a/TODO.md b/TODO.md
index 7e77fb3..03b7b68 100644
--- a/TODO.md
+++ b/TODO.md
@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- the `Dockerfile` build stage keeps the Go module cache out of `builder`'s
+  home and copies the sources with `--chown`, so no `chown -R` walks them
+  (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/43)
+
 - `--version` prints `sfdupes VERSION` to stdout; README documents it and
   `--help` (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/15)