Cache from simplification

[toote]

Reduced the complexity of the cache-from option. Now it is just a pass-through to the corresponding options in docker compose. This means that:

• images will not be pulled by the plugin (docker should take care of that)
• there are no more groups
• the option format simplification makes it possible to support all formats available in the underlying docker engine

[tomowatt]

@toote are you able to assist with the conflicts here? I've tried fixing them up locally e.g.,

git checkout toote_build_simplification  
git checkout toote_cache_from 
git merge toote_build_simplification  

But running the tests locally ends up with a lot of failures

The merges of the previous PRs worked fine with the tests but hesistant to push anything in case of messing more up

[toote]

@toote are you able to assist with the conflicts here? I've tried fixing them up locally e.g.,

But running the tests locally ends up with a lot of failures

The merges of the previous PRs worked fine with the tests but hesistant to push anything in case of messing more up

Apparently, I forgot to do a mergeback in the pile of PRs I created which caused these conflicts. So sorrry :(

I have resolved them all and confirmed that there shouldn't be any more in the remaining PRs.

While I was at it, I had to review a lot of tests so I did some refactoring to remove around 500+ unnecessary lines :)