etcd_docker 1: Refactor resources/docker package into resources/dockerexternal, resources/dockerm3, x/dockertest (#4145)

[andrewmains12]

PR 1 of etcd test refactor

I am introducing a docker based etcd test framework. Overall structure I'm going for:

resources/dockerm3 -- m3 docker containers resources/dockerexternal -- dependencies (prometheus, etcd)

The reason I need this refactor: I'm going to pull the dockerexternal package into a bunch of internal tests. This introduces circular dependencies with the M3 resources, thus splitting out here.

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Stack:

• #4154
• #4153 ⬅

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.