Use docker parsing when manipulating dockerfiles

[bparees]

currently build/builder/docker.go does manual parsing of the dockerfile for some manipulation. we should use docker libs instead for better safety.

@smarterclayton fyi.

[mfojtik]

@Kargakis https://github.com/docker/docker/blob/640e0fc578e5acdaa71216a300396dfa9af8054f/builder/parser/parser.go

[smarterclayton]

In general, we also need more test cases and coverage on this area (if we're rewriting Dockerfiles)

----- Original Message -----

@Kargakis
https://github.com/docker/docker/blob/640e0fc578e5acdaa71216a300396dfa9af8054f/builder/parser/parser.go

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[0xmichalis]

Track process here: #1184