Create a GitHub Action to run E2E tests

[lb324567]

Describe the new feature you'd like Currently E2E testing is done manually, if it's done at all. E2E tests should be run when changes are made to be put into the Gaffer repository. The code for the E2E tests currently lives in Gaffer-Docker, but a GitHub Action would be able to clone this repo and run the tests automatically.

Why do you want this feature? This allows us to ensure that the code works fully and hasn't broken any current functionality.

Additional information

• This can be created in Gaffer Docker to call the Gaffer branches, or within Gaffer to call the Gaffer Docker branch.
• This is done using Gaffer Docker so that we can use the containers to run tests against a more realistic environment (with Accumulo etc)
• E2E tests have a larger overhead so to reduce this it can be a manually trigger action that should be run BEFORE a PR is accepted but only needs to be run once if it passes