Hadolint is in the process of adding the severity of a lint rule to the
commandline output: https://github.com/hadolint/hadolint/pull/501
This change utilizes that to show the severity in vim.
* Add documentation for hadolint (doc/ale-hadolint.txt)
* Allow `hadolint` linter to run via docker image
These changes enable the `hadolint` linter to run via the author's
docker image, if present. Three modes are supported:
* never use docker;
* always use docker; and
* use docker as a failback.