Include build-tools in build image

[jml]

My understanding is:

• the base build images provide the basic environment we need for a "standard" weaveworks build
• the build-tools repo provides the basic tools we need for same

However, the tools in build-tools can depend on specific versions of things. Often, spellcheckers and linters introduce new checks in new versions, breaking existing code.

It would be ideal if the build-tools tools could be provided as standard in the build images. That way, we wouldn't need to subtree this repo in, and we could be sure that the dependencies were the right ones for the tools.