NOTE: The custom-linting-rules test fails due to the following (legit)
warnings:
ale_linters/clojure/clj_kondo.vim:29 Use snake_case names for linters
ale_linters/elixir/elixir_ls.vim:15 Use snake_case names for linters
ale_linters/go/golangci_lint.vim:54 Use snake_case names for linters
ale_linters/swift/swiftformat.vim:56 Use snake_case names for linters
The message wasn't getting printed because docker was explicitly only
being asked to connect stdout (ignoring stderr). Unclear yet why the
error code wasn't getting bubbled up.
