diff --git a/linux/build/scripts/build.sh b/linux/build/scripts/build.sh
index d7bbcbc..df6a63a 100755
--- a/linux/build/scripts/build.sh
+++ b/linux/build/scripts/build.sh
@@ -27,7 +27,12 @@ cd "${REPO_ROOT}"
 # --- Pinned builder image ---------------------------------------------------
 # Bumped together with linux/build/docker/Dockerfile.builder. The digest form
 # is required; refusing the tag-only form is what stops a silent host drift.
-BUILDER_IMAGE="${BUILDER_IMAGE:-docker-registry:5000/silvermetal-builder@sha256:REPLACE_WITH_PUSHED_DIGEST}"
+#
+# Default points at the internal registry hostname used by CI runners. The
+# external endpoint (docker-registry.silverlabs.uk) serves the same registry
+# and the same content-addressed digest, so external dev can override with:
+#   BUILDER_IMAGE=docker-registry.silverlabs.uk/silvermetal-builder@sha256:cedef039425e0b0f5901c1023eda820c7aa38ab4b81c2bb1e12d64cadb3d6c85
+BUILDER_IMAGE="${BUILDER_IMAGE:-docker-registry:5000/silvermetal-builder@sha256:cedef039425e0b0f5901c1023eda820c7aa38ab4b81c2bb1e12d64cadb3d6c85}"
 
 if [[ "${BUILDER_IMAGE}" != *"@sha256:"* ]]; then
     echo "build.sh: BUILDER_IMAGE must be pinned by digest, got: ${BUILDER_IMAGE}" >&2