Add docker build --pull command.

[glogiotatidis]

This waits for docker/docker-py#624

[glogiotatidis]

Interestingly docker-py defaults to pull=True. I suggested that this gets changed to match docker cli docker/docker-py#622.

Even more interestingly pull=True does not work. I reported the issue here moby/moby#13631.

[dnephin]

Resolves #726 and will replace #731

[dnephin]

Looks like the latest build is reporting a syntax error, but I don't see it in the diff. If you have a chance to rebase this on master, it would be good to get this in.

[thaJeztah]

I see tests are still failing;

compose/service.py - Could not parse ast

    Traceback (most recent call last):
      File "/tmp/.pre-commit/repo7MclqR/py_env-default/lib/python2.7/site-packages/pre_commit_hooks/debug_statement_hook.py", line 38, in check_file_for_debug_statements
        ast_obj = ast.parse(open(filename).read(), filename=filename)
      File "/usr/local/lib/python2.7/ast.py", line 37, in parse
        return compile(source, filename, mode, PyCF_ONLY_AST)
      File "compose/service.py", line 725
        pull=pull,
    SyntaxError: keyword argument repeated

Fix End of Files.........................................................Passed
Flake8...................................................................Failed
hookid: flake8

compose/service.py:725:1: E901 SyntaxError: keyword argument repeated
tests/integration/cli_test.py:116:6: F821 undefined name 'patch'

[glogiotatidis]

@thaJeztah code updated. I still get errors but I need a hint to fix them.

[dnephin]

@glogiotatidis don't worry about this error, we have it ticketed as #1922

LGTM

[dnephin]

Done in #2041