relx, jsx and actual bugfix

So, I don't expect you to want most of these changes necessarily. But I thought I'd include them to get your thoughts, and I can pull them out if you do want some of these commits.

The only bug fix is the last one, `erldocker_api:post_body` doesn't exist, I replaced the call to it in `docker_image` and `docker_container` with `erldocker_api:post`.

I replace jiffy with jsx solely, in this case, because otherwise building requires building C code.

I added `relx` for building a release so it can be easily built and started properly. Which I also replaced the `Makefile` because of. Which is probably the biggest issue since you had your sync stuff in there, I didn't get that working so I haven't added it back.