Yes. We build many container base images for all architectures. have a browse and pick what works for you -- https://hub.docker.com/r/resin/ and also http://docs.resin.io/runtime/resin-base-images/

ARM isn't x86. You get a nice generic kernel with enough built in drivers plus an Initrd and it can boot on most Intel/AMD hardware both old an new (at least the Pentium 4+..maybe even the P2 or P1. Haven't tried a newer distro on them).

ARM isn't anywhere near as standardized. There are usually vendor kernel patches, not just for drivers, but dependent on the block layout of that particular ARM board. uBoot is often forked and also specialized. There is no one shoe fits all Grub.

Yes - they would have to be built with ARM base images.