Container Support

https://pulp.plan.io/issues/8629

As a result docker-content-digest response header contains only the hash of sha256 instead of sha256: <hash> string.

This leads to pulp3 to pulp2 failure and docker pull fails with older clients.

The issue was introduce with this commit https://github.com/pulp/pulp_container/commit/0a362e236bcf2d5ecac15afd4d8d5d166f732637#diff-c035f88089448e23039b5c302d6d17d169a92cfbdc325c75a0ddf3fd7e3a46b4R286

$ http http://pulp2-nightly-pulp3-source-centos7.lan/pulp/container/ocp3/manifests/v3.7.42-2
HTTP/1.1 200 OK
Connection: Keep-Alive
Content-Length: 5405
Content-Type: application/vnd.docker.distribution.manifest.v1+prettyjws; charset=utf-8
Date: Mon, 26 Apr 2021 21:56:20 GMT
Docker-Content-Digest: fc8071636efc3f654d50b4e7f1dea9fdc1face1a30c7f5d42665c964666dbca
Docker-Distribution-API-Version: registry/2.0
Keep-Alive: timeout=5, max=10000
Server: Python/3.6 aiohttp/3.7.4