Problems with the "Using Docker With GitHub Actions" example

[jcohen02]

The Using Docker with GitHub Actions example has some problems that need resolving:

1. The currently provided YAML for the Actions configuration appears to fail to run as a result of a permissions error (this is the problem highlighted in issue #220).
2. The Settings part of the GitHub UI seems to have been modified since the example was created and some of the process described doesn't match up with what users will see on the GitHub website.
3. The formatting of the YAML is broken - presumably as a result of the switch to the Workbench framework, this needs fixing.
4. There are various references to GitHub.io, I think the correct terminology to use here is GitHub Pages and it probably makes sense to switch to this to help learners if they're searching online or in the GitHub docs for further information.

Further details

The fix for (1) is detailed in #220 and will be incorporated in a PR that addresses changes highlighted here.

Addressing (2) should simply be a case up updating the description in the example to match the current interface structure.

(3) should be fixable by changing the text formatting configuration to work with the new workbench styles.

Finally, (4) will require some minor rewording.