Index and Upstream should be mutually exclusive

[kofalt]

While #20 and #30 fixed the most glaring issues about configuration, one bit is still a little confusing: mixing the maintenance of two images into a single file.

Two operations are expressed in the same file with the upstream and index keys:

1. Pulling an image down from the index and storing it in the graph
2. Taking that stored image and mutating it

Instead, let's separate these into two files, each concerned with a single image.

Before:

[image]
    name     = "docket.polydawn.net/ubuntu/12.04"
    upstream = "index.docker.io/ubuntu/12.04"
    index    = "ubuntu:12.04"

After, ubuntu folder:

[image]
    name     = "docket.polydawn.net/ubuntu/12.04"
    upstream = "index.docker.io/ubuntu/12.04"

After, ubuntu-index folder:

[image]
    name     = "index.docker.io/ubuntu/12.04"
    index    = "ubuntu:12.04"

Docket should reject a configuration file that has both upstream and index keys. This carries the new restriction that an image can be sourced from an index, or a non-index source, but not both.

Hopefully, this restriction makes sense: you're either maintaining your own history of an image that came from an unversioned index, or you're not.

An advantage of this new split is we can now have build steps dedicated to maintaining index images! In the case of Ubuntu, this is rather pertinent: we can now clean out useless apt-get state from the index image before saving it to the graph, saving space. This is optional, but could be useful for performing tune-up on other people's index images.

[kofalt]

Finished in a8408c3.

[kofalt]

As a side note, this shaved 34 MB off graphs tracking index ubuntu:

$ docket build
60M

$ docket build --noop
94M