Add python additionalFiles option

[wagoodman]

Here's a rough cut at issue #2... once there is general agreement on this approach, I'll fill in some tests and start filling this out for other languages.

This example isn't working yet since the metaparticle lib within the container is unaware of the new functionality (specifically, the PackageFile is seen as a bad import)... otherwise this example appears to be working.

I decided to go with an class over a simple dictionary of { dest: src} because now it is

• a bit more testable (todo: add test for this :) ),
• allows for extra options such as mode very easily without refactors later, and
• allows any validations or other logic to be confined to the class if necessary.

However, I think it would be a good time to discuss having mixed objects in the Containerize definition or to instead always use dict/list primitives when possible... e.g. something like this instead:

@Containerize(
    package={
        'name': 'file-example',
        'repository': 'docker.io/brendanburns',
        'publish': False,
        'additionalFiles': {
            './data.json': '/some/container/path',
            './get_data.sh': '/another/container/path',
        }
    }
)

[brendandburns]

Needs a rebase since I merged your other PR...

[wagoodman]

@brendandburns : rebased. Do you have any opinions around providing a list of files via a list of rich objects (implemented in the PR) or by instead a simple dictionary (example given in the description)?

[brendandburns]

I think that nested is great actually, it looks more expressive to me.

Once this is fully ready and working, I'd be ok to merge.

Thanks! --brendan

[wagoodman]

Just to double check, @brendandburns, by "nested" do you mean the alternative example above:

@Containerize(
    package={
        'name': 'file-example',
        'repository': 'docker.io/brendanburns',
        'publish': False,
        'additionalFiles': {
            './data.json': '/some/container/path',
            './get_data.sh': '/another/container/path',
        }
    }
)

or do you mean what's implemented in this PR? e.g.:

@Containerize(
    package={
        'name': 'file-example',
        'repository': 'docker.io/brendanburns',
        'publish': False,
        'additionalFiles': [
            PackageFile(src='./data.json', dest= '/some/container/path', mode='0400'),
            PackageFile(src='./get_data.sh', dest='/another/container/path'),
        ]
    }
)