Updates to docker build

[mattjohnson]

This pull request makes the following changes:

• Builds docker image based off of working directory with pinned version of maven using jdk-8
• Adds step to travis to build image to confirm it builds successfully.

It relates to the following issue #s:

• Fixes https://github.com/sonatype-nexus-community/nexus-blobstore-google-cloud/issues/27