Release and build improvements

[fntlnz]

In #39 @BondAnthony made possible to have automated releases using goreleaser.
I'm opening this issue to follow up some improvements on that area so that we don't forget what's left to do:

• Since we want to test it for a while before adding it to CI, we are now running goreleaser locally when tagging. The CI does the release build for every PR and merge but does not do the release process. It's manual in the sense that a human executes the command that does the automatic release not in the sense that a human has to do the whole release process. We want to have this 100% automated via travis, see the goreleaser doc.
• The goreleaser is not building the docker images right now, we want to do that. Also, since it is building the binaries, we want to use the built ones instead of rebuilding them again inside the docker images (I'm talking about 'tracerunner' here).
• The released binaries now use the bpftrace image tagged as latest. They must use the one with the same commit has that we built with Travis when that commit has was published.

[BondAnthony]

I should have a Pr open over the next couple of weeks to handle the image build process. Commit tag should be pretty easy to incorporate with templating. I will link the Pr once it’s open.