Wrap the gate comment where its lines wrap

Dockerfile

@@ -1,7 +1,8 @@
-# The merge gate: `docker build .` fails if vet, complexity, formatting or tests fail, and CI
+# The merge gate: `docker build .` fails if vet, complexity, formatting or tests
-# runs exactly this. Day-to-day, prefer `docker compose run` (bind-mounts source,
+# fail, and CI runs exactly this. Day-to-day, prefer `docker compose run`
-# no rebuilds). GO_VERSION defaults to the latest stable Go; the lowest supported
+# (bind-mounts source, no rebuilds). GO_VERSION defaults to the latest stable Go;
-# version builds the `portable` stage, which every supported Go must pass:
+# the lowest supported version builds the `portable` stage, which every supported
+# Go must pass:
 # docker build --build-arg GO_VERSION=1.22.12 --target portable .
 ARG GO_VERSION=1.27.1
 FROM golang:${GO_VERSION} AS portable