Add containerized pre-commit hook

[lhriley]

Simple PR that adds support for using the pre-build Docker container image rather than requiring users to install the binary or build the golang package from source.

Tested locally and works with this .pre-commit-config.yaml:

---
repos:
  - repo: https://github.com/Labelbox/helm-docs
    rev:  65f1d83 # add-docker-to-pre-commit-hooks
    hooks:
      - id: helm-docs-container
        args:
          # Make the tool search for charts only under the `charts` directory
          - --chart-search-root=charts

Related:
pre-commit/pre-commit#2968

[Nepo26]

Hm seems nice. Will look into it by the end of the next week.

[Nepo26]

If possible also add to the README the description of how to define the use of the container and how it should be ran.

[lhriley]

@Nepo26 I pushed up changes to the README as requested. There were some whitespace issues that were cleaned up by my editor that makes the diff a bit messy.

[Nepo26]

Just tested here and didn't see any problem.

Thanks @lhriley - looks great :)