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
I guess we should use it for now, but if we encounter any instabilities I guess that's actually something we should report back to the go project.
:+1: