Use docker image for custom-check scripts

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.

test/script/check-tag-references

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+set -e
+
+exit_code=0
+tag_regex='[gb]\?:\?\(ale\|ALE\)[a-zA-Z_\-]\+'
+
+tags="$(mktemp -t tags.XXXXXXXX)"
+refs="$(mktemp -t refs.XXXXXXXX)"
+# Grep for tags and references, and complain if we find a reference without
+# a tag for the reference. Only our tags will be included.
+grep --exclude=tags -roh "\\*$tag_regex\\*" doc | sed 's/*//g' | sort -u > "$tags"
+grep --exclude=tags -roh "|$tag_regex|" doc | sed 's/|//g' | sort -u > "$refs"
+
+exit_code=0
+
+if ! [[ $(comm -23 $refs $tags | wc -l) -eq 0 ]]; then
+  exit_code=1
+fi
+
+rm "$tags"
+rm "$refs"