Allow `hadolint` linter to run via docker image (#720)

* 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.

1 files changed, 37 insertions, 0 deletions

diff --git a/doc/ale-dockerfile.txt b/doc/ale-dockerfile.txt
new file mode 100644
index 00000000..e87a3aa7
--- /dev/null
+++ b/doc/ale-dockerfile.txt
@@ -0,0 +1,37 @@
+===============================================================================
+ALE Dockerfile Integration                             *ale-dockerfile-options*
+
+
+-------------------------------------------------------------------------------
+hadolint                                              *ale-dockerfile-hadolint*
+
+  hadolint can be found at: https://github.com/lukasmartinelli/hadolint
+
+
+g:ale_dockerfile_hadolint_use_docker     *g:ale_dockerfile_hadolint_use_docker*
+                                         *b:ale_dockerfile_hadolint_use_docker*
+  Type: |String|
+  Default: `'never'`
+
+  This variable controls if docker and the hadolint image are used to run this
+  linter: if 'never', docker will never be used; 'always' means docker will
+  always be used; 'yes' and docker will be used if the hadolint executable
+  cannot be found.
+
+  For now, the default is 'never'.  This may change as ale's support for using
+  docker to lint evolves.
+
+
+g:ale_dockerfile_hadolint_image               *g:ale_dockerfile_hadolint_image*
+                                              *b:ale_dockerfile_hadolint_image*
+  Type: |String|
+  Default: `'lukasmartinelli/hadolint'`
+
+  This variable controls the docker image used to run hadolint.  The default
+  is hadolint's author's build, and can be found at:
+
+    https://hub.docker.com/r/lukasmartinelli/hadolint/
+
+
+-------------------------------------------------------------------------------
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl: