Fix read offset check for inmemory driver

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>
Merge pull request #787 from ahmetalpbalkan/next-generation

NG: Fix read offset check for inmemory driver
Correct ENV declaration in Dockerfile
Merge pull request #793 from stevvooe/update-go-version

Correct ENV declaration in Dockerfile
Address race condition in client tests (closes #784)
Merge pull request #792 from stevvooe/next-generation

Initial implementation of Manifest HTTP API
Merge pull request #794 from stevvooe/ng-client-race-condition

Address race condition in client tests (closes #784)
Create copy of buffer for SignedManifest.Raw

Without this copy, the buffer may be re-used in the json package, causing
missing or corrupted content for the long-lived SignedManifest object. By
creating a new buffer, owned by the SignedManifest object, the content remains
stable.
Use json.MashalIndent for raw manifest json

This provides compatibility with what is in docker core, ensuring that image
manifests generated here have the same formatting. We'll need to automate this
some how.
Cleanup image verification error handling

This diff removes a few early outs that caused errors to be unreported and
catches a missed error case for signature verification from libtrust. More work
needs to be done around ensuring consistent error handling but this is enough
to make the API work correctly.
Avoid manifest verification errors by using Raw

Because json.Marshal does compaction on returned results, applications must
directly use SignedManifest.Raw when the marshaled value is required.
Otherwise, the returned manifest will fail signature checks.
configuration.Parse should take io.Reader
Add registry main cmd
Add TODO about manifest tampering test
StorageDrivers should exit non-zero on error (closes #803)
Merge pull request #805 from stevvooe/ng-manifest-bugs

NG: Various Manifest Oriented bugfixes
Merge pull request #807 from stevvooe/ng-nonzero-exit-error

StorageDrivers should exit non-zero on error (closes #803)
Merge pull request #806 from stevvooe/ng-registry-main

NG: Add registry main cmd