[Proposal] Add S3 use-sigv4 Configurable

[schmitch]

Currently the AWS S3 Auth sigv4 isn't supported with the docker-registry. So any new Bucket that is created on the new AWS Datacenters aren't supported.

My suggestion is, to introduce AWS_SIGV4 environment variable which could be either True or False and is False by default (the current).
Also AWS_SIGV4 could only be SET, when AWS_HOST is set, it won't work without since the SIGV4 needs an host to be specified.

The other route would be to automatically set SIGV4, when the user sets the HOST via AWS_HOST.

Both solution are currently blocked by #674.

[dmp42]

@sday @wking @chuegle what do you think?
I'm ok with the extra env var

[chuegle]

I believe you can just set:
-e S3_USE_SIGV4=1
and tell boto directly to use sigv4 for s3.

Another idea may just be to make it clear/easier to inject a boto.cfg into the default registry container.
e.g.

make_boto_config > boto.cfg
docker run -v /boto -v $PWD:/mnt --name boto_cfg busybox cp /mnt/boto.cfg /boto/boto.cfg
docker run ... --volumes-from boto_cfg -e BOTO_CONFIG=/boto/boto.cfg docker-registry

[dmp42]

+1 for -e S3_USE_SIGV4=1 (needs to be documented).

Savvy people can still do some acrobatic stuff with boto.cfg, but I don't want to promote that as the main way to go (although, I'm ok adding some infos about that into ADVANCED.md).

@c-schmitt yours to take if you want :-)

[schmitch]

@dmp42 yeah I will do it on weekend

[schmitch]

I created a pull request at #705 currently it will fail hard if no AWS_HOST/boto_host is specified.
It's configurable via -e AWS_USE_SIGV4 or in the config file with s3_use_sigv4.
Other variables are alled like this, too so I used the same scheme.