Add support for every build context type supported by the daemon

[moysesb]

This is a Proposal PR.

Implements #1209 (and more) using some of the ideas from docker/docker-py#538

This changeset adds support for specifying the build path in any of the formats supported by docker-py's implementation of the build() operation. I expanded the section on build: in docs/yml.md with a brief explanation of what kind of values are supported. The implementation is mostly in a new file: compose/context.py.

[thaJeztah]

Nit; indentation looks off

[moysesb]

ping @aanand. Any word on whether this is being considered for merging? Thanks!

[aanand]

Thanks for the PR! I'm theoretically in support of the feature, but:

• Why are we doing LZMA decompression client-side? Is that how the Docker CLI does it? I'm not relishing the bloat there.
• A lot of this logic looks like it belongs in docker-py. I see from Design proposal: make the docker-py interface more 'data oriented' docker-py#538 that it's non-trivial to overhaul build and create_container at this time, but the logic in compose.context could at least be moved to docker.utils.
• I deeply appreciate your including documentation. I don't think it needs to be that comprehensive, though - we should instead make sure the docker build docs cover everything and link to them, and then show a short list of example values for build.

[moysesb]

@aanand

• The lzma thing is just a workaround for the fact that python2's tarfile module doesn't support the lzma format. The rationale is that compose should (perhaps) perform a preliminary sanity check on the archive before submitting it to the daemon, since the upload can be expensive if the file is large. The only way to make sure it's a valid archive with python2 is to include a check using the lzma module. Note that there's no actual decompression on the client side, just a peek to see if the tarfile/lzma module recognizes the archive format.
• I agree that most of this logic should be in docker-py. I'll wait for the maintainers to review
  Design proposal: make the docker-py interface more 'data oriented' docker-py#538 and then move context.py there.

[moysesb]

@aanand Moved context.py to docker-py and pushed a new commit here. I understand there's a discussion going on about how much validation should be performed by compose for the value of build (in #1372) - the implementation of create_context_from_path() in my docker-py fork does validate the supplied path, so maybe that should be taken into account.

[aanand]

How widely installed is liblzma? We should make sure it exists on all the main already-supported Linux distros and OS X.

[moysesb]

Oh, this is a leftover from when context.py was in compose, there's no actual need for this dependency here. The lzma compression is handled as a special case in docker-py now: under py3 the tarfile module has built-in support for lzma (presuming the xz-utils headers were available when interpreter was built) but under py2 docker-py just accepts a file with an '.xz' extension as a lzma compressed tarball and passes it on to the build call without 'peeking' into it.