How well is the caching supported. Is there a way for me to cache and restore my gem dirs?
CircleCI has great caching support for gems, etc (we've had this for about 2 years, so it's really mature). I suspect it might need to be tweaked to support docker use cases a bit better, so can you try it out and let us know?
Gem caching in docker is more about layer construction than anything the CI would do as such.