Cache Docker images

[japaric]

Probably hard. There's no Travis support for this AFAIK.

[kamalmarhubi]

I'm working on containy-thing for pretty much exactly this use case. Benefits: no sudo needed so we run on container infra (shorter queues), and should be able to cache images (they'll just be tarballs). It's probably a few weeks out, but wanted to drop a note here.

[japaric]

Looks interesting! I'll give it a try when I get time.

Something that trust needs is to build on top of Ubuntu releases (e.g. Precise, Trusty, etc. or any other distro release) and it seems that the ROOTFS argument takes care of that. The other thing that trust needs is read-only mounts, containy-thing doesn't seem to support that AFAICT.

they'll just be tarballs

Some examples would be nice! I seem to never get the permissions right when I tarball rootfses 😄.

[japaric]

we are now using [cross](https://github.com/japaric/cross] which provides pre-built docker images. There's no longer a need for this.