Publish Docker image to allow users the ability to pull a container in Github actions

[TheQueenIsDead]

In response to this StackOverflow question and #42, publishing a Docker image for users of this action will allow them to directly pull the image instead of having to build the source code each time. This reduces the time to prepare the action from ~30 seconds to ~8

To enable this change to work, you will have to create a new repository in DockerHub, and add your username and password to the Secrets configuration in GitHub

It's worth noting that you can publish an image to the GitHub repository if you wish, and the instructions for that can be found here:
https://docs.github.com/en/actions/guides/publishing-docker-images

To view an example of what this change enables, please see the changes made in this PR in conjunction with the DockerHub image, and an example use case:

Example Usage: https://github.com/TheQueenIsDead/action-caching/blob/main/.github/workflows/push.yml
Example Repo Workflow: https://github.com/TheQueenIsDead/action-caching/runs/3259818711?check_suite_focus=true
(Please be aware the example workflow failed, but after the fact, and seemingly due to the fact that my commit message includes characters that are perhaps not being handled. The Setup Job and Pull thequeenisdead/gh-action-bump-version steps are the most important things to look at)

[TheQueenIsDead]

@phips28 , please close this if you're going to be going down the route of an NPM action instead of a Dockerised one 👍

[phips28]

#118