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.
sed wasn't using -E, so '|' wasn't being handled properly. Seems likely
that's sed-implementation specific, so now it runs through docker's sed
to support portability.
This makes some of the run-test output less misleading.
Also fix a minor shellcheck issue: "\*" and "\\*" are equivalent, but
the second one makes clear that the literal backslash is intentional.
