Cosmos-sdk: Autobuild minimal Docker image on releases

[cwgoes]

Requested by a user - https://github.com/cosmos/cosmos-sdk/pull/1186#issuecomment-397072129. We already have a Dockerfile. On each release we could build 'cosmos/gaia' and tag it with the release version. We can still encourage users to build from source themselves, but Docker is a helpful additional option (and convenient for cloud deployments of e.g. sentry nodes).

cc @greg-szabo - thoughts?

[greg-szabo]

That's exactly what we're doing with Tendermint. I couldn't create the "cosmos" organization (or repo? I'm confused by these names) in Docker Hub though, because it's already owned by someone. We can do tendermint/gaiad and tendermint/gaiacli for now.

Also, tendermint automatic releases create the fresh containers but last time I discussed this, the agreement was that we're not shipping binaries for Cosmos SDK, hence there's no automation there.

[jackzampolin]

I've talked to @ebuchman and @adrianbrink about this and I'm working on it later today. I'll jump in the SRE meeting with @greg-szabo and @zramsay to chat about strategy here.

[cwgoes]

I don't think this belongs in Next Release (but this is still a good idea IMO).

[jackzampolin]

We are tracking this in the devops repo now.