docs: correct stale script headers and record the config-verify cost (closes #137)

script/bootstrap credited the goimports pin to script/fmt-check, which
runs gofmt only; the header now credits script/fmt. script/cibuild
still described the Dockerfile as running make check, which stopped
being true once linting moved to its own stage. The docker-missing
warning in script/bootstrap is one sentence instead of three fragments
each carrying the bootstrap: prefix. Dockerfile.lint now states the
residual risk of skipping golangci-lint config verify: unknown
top-level keys in .golangci.yml are ignored silently, so a mistyped key
lints clean and applies nothing.

Comment and message text only; no behaviour changes.

script/cibuild

@@ -1,6 +1,7 @@
 #!/bin/sh
-# script/cibuild: run the CI build. The Dockerfile runs make check, so
-# a successful build implies all checks pass.
+# script/cibuild: run the CI build. The Dockerfile's lint stage runs
+# make fmt-check and golangci-lint; its builder stage runs make test
+# and make build. A successful build implies all of those passed.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"