Run the linter only in Docker (closes #20)

script/lint now builds the new Dockerfile.lint, whose last step runs
golangci-lint. The build cache is off, so every run executes the
linter, and no image is kept. The Dockerfile lint stage calls
golangci-lint directly, since make lint is now a docker build of its
own. script/fmt no longer runs golangci-lint --fix, and
script/bootstrap no longer installs it. golangci-lint config verify is
left out: it fetches its schema over the network, unpinned. The README,
TODO.md and the script/cibuild comment say what now runs.

Model: opus-5-5

script/cibuild

@@ -1,6 +1,6 @@
 #!/bin/sh
-# script/cibuild: run the CI build. The Dockerfile runs script/check, so
-# a successful build implies all checks pass.
+# script/cibuild: run the CI build. The Dockerfile runs the format check,
+# the linter and the tests, so a successful build means they all pass.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"