Move to own dev image

Moving away from brad image into our own.
The dockerfile for that dev-image is also provided.
Benefits include:
 * faster builds (since go get deps are now bundled in the base image)
 * better control over exactly what we run on top of
 * easier to get different versions of go
 * simpler drone file
 * way toward more unified bases tools for different projects
Let's bet on 1.4. You sell it to the powers. If they are not happy, you can still roll your own with 1.3 (I guess you will ship a custom Dockerfile anyhow). After-all, this is only a drone-test image.