Develop GitHub workflow to allow easy review of PRs

[Luke Mondy]

We need to enable github workflows that will allow contributors and reviewers to be able to easily view the changes they would like to make to the seL4 website.

Things to do:

• Create a Dockerfile that can setup Apache to work with the seL4 website, and host it successfully. #1
• Get the GitHub workflow to override the /var/www/seL4 dir within the container with whatever the PR state is.
• Use wget to snapshot the website from the locally run server
• Commit the files to another repo
• Make a link using the htmlpreview service, and comment it on to the PR
• Clean up the gross bits

[Gerwin Klein]

It's probably easier to let the github workflow mount as default and then set a link inside the container from /var/www/seL4 to $GITHUB_WORKSPACE when it starts up.

[Luke Mondy]

This has been done in:
#3
#5
#6
#7
#8