After the discussion in #116, this stores the blob content types
in extended attributes when using the filesystem backend.
If the underlying filesystem doesn't support extended attributes,
storing blobs won't work; also, if extended attributes get removed,
blobs won't be served anymore. We can relax this behavior if
needed (i.e. log errors but still accept to store or serve blobs).
However, since the Docker Engine (and possibly other container
engines) won't accept to pull images from a registry that doesn't
use correct content types for manifest files, it could be argued
that it's better to give a hard fail. (Otherwise, the container
engine gives cryptic error messages like "missing signature key".)
I can change that behavior (and log errors but still store/serve
blobs to the filesystem) if you think it's better.
In case the `GOOGLE_APPLICATION_CREDENTIALS` environment variable is not
set, a redirect to storage.googleapis.com is issued, which means the
underlying bucket objects need to be publicly accessible.
This wasn't really obvious until now, so further clarify it.
Extends storage.Persist to accept a Content-Type argument, which in
the GCS backend is persisted with the object to ensure that the object
is served back with this content-type.
This is not yet implemented for the filesystem backend, where the
parameter is simply ignored.
This should help in the case of clients which expect the returned
objects to have content-types set when, for example, fetching layers
by digest.
This gets rid of the package called "server" and instead moves
everything into the project root, such that Go actually builds us a
binary called `nixery`.
This is the first step towards factoring out CLI-based functionality
for Nixery.
