create first pass Dockerfile(s)

Create a base Dockerfile that builds nsq and all its subdirectories.

Create a Dockerfile for nsqd that inherits the base and will run nsqd
with the passed command line flags.

Create a Dockerfile for nsqlookupd that inherits the base and will run
nsqlookupd with the passed command line flags.

Create a Dockerfile for nsqadmin that inherits the base and will run
nsqadmin with the passed command line flags.

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I just opened this pull request so the work in progress lived somewhere besides a gist and people could find it and see the status easily. See more discussion in #381.