context

Context helpers inspired by https://godoc.org/github.com/docker/distribution/context.

Documentation

Differences:

1. Package github.com/satori/go.uuid used for uuid generation instead of github.com/docker/distribution/uuid
2. Package github.com/apex/log used to log instead of github.com/Sirupsen/logrus
3. gorilla mux extracted because I don't need it.