Fix edge cases in build arg processing

[jakehow]

With #828 recently merged I tested in some Dockerfiles where we are using build args.

The current implementation misses some functionality and does not handle a couple of edge cases mostly detailed in the documentation here https://docs.docker.com/engine/reference/builder/#arg

• Build Args support default values: ARG <name>[=<default value>]
• Docker parses instructions even when there is more than one space between the command and the value. EX: ARG FOO is valid
• Usage of the build arg can be done with curly braces eg: COPY ${FOO}

There are also two additional issues that I didn't have in my files, but should be handled to match the behavior of docker build... that are not currently handled by this PR

• Inline defaults: COPY ${FOO:-server.go}

• Args are scoped to replace values only below where they are declared in the Dockerfile:
  For example if we have a build arg FOO set to start

  FROM ubuntu:14.04 
  COPY ${FOO:-server.go} .
  ARG  FOO
  CMD $FOO
  

  will evaluate to:

  FROM ubuntu:14.04
  COPY server.go .
  ARG  FOO
  CMD start
  

Additional tests have been added for the variations handled by this PR and pass for me.

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[jakehow]

@googlebot I've signed the CLA

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CLAs look good, thanks!

[dlorenc]

Thanks for the fix and detailed test cases!