Add support for releasing an offical docker image

[rogchap]

This adds a docker image plus the relevant docker goreleaser/travisci configuration.

To test locally run:

goreleaser --snapshot --skip-publish --rm-dist
docker run vektra/mockery --version

To test more than just the version you will need to mount your volume:

docker run -v "$PWD":/src -w /src vektra/mockery -all

Note: I originally used a Scratch base image, which puts the image around 6MB... but mockery makes use of the go command (go list etc), so I need to use an image with Go installed. This bumped the image to 375MB!! It's not a major problem as most people will have the Golang base imaged cached, but this could slow down peoples CI for downloading a larger image.
2 future improvements would be to remove the need for executing go commands and installing Go onto the scratch image rather than using alpine.

@LandonTClipp We will need to add the secrets for travis: DOCKER_USERNAME and DOCKER_PASSWORD

I have presumed that there is/would be a docker repo for user vektra? We can change if needed.

I have not updated the README, as I think it would be best to get this CI pipeline confirmed working then update to suggest docker. I would recommend that for CI people use the docker version.

Closes #180

[LandonTClipp]

Looks fantastic, I will merge this in the US morning tomorrow once I get the docker credentials figured out. I don't believe we have an existing login for docker so I will get that created.

[LandonTClipp]

I need to get added to the vektra dockerhub org before we can merge this, I've emailed @evanphx to get access to it.