Dockerized manager build script

[davidvossel]

This introduces an optional path for building the capk manager binary using a standardized docker container. This allows us to optionally decouple the golang build version from whatever is running locally on our dev machines.

running the following will use a golang base container to build the manager binary and dump it into the src tree's bin/manager directory.

CAPK_DOCKERIZED_BUILD=true make manager

The existing make manager without usage of the CAPK_DOCKERIZED_BUILD env var remains unchanged, and will use the local go environment.

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[cchengleo]

This is another PR #46 trying to solve the issue differently.

[cchengleo]

@davidvossel and @rmohr, what're your thoughts regarding the make manager target? Are we prefer to use volume or production image layers to cache for dependencies?