Add support for dep

[VeganBroccoli]

• This PR adds dep support to the project. The dockerfile also uses dep to ensure that it can succeed in building.
• Updated dockerfile image to alpine 3.7.

Note that an override had to be specified in the Gopkg.toml file to keep Docker working with Go 1.9. This is due to go-dockerclient using master of Docker in their Gopkg.toml. This override is unnecessary for Go 1.10, but only apline:edge has packages for Go 1.10.

[josegonzalez]

Seems like this fixes local builds. Thanks for the pull request!