* Use registry-1.docker.io for Docker Hub's docker.io host
docker.io doesn't serve the registry API: https://docker.io/v2/ redirects
to https://www.docker.com/, which URLSession follows, getting back an HTML
page with HTTP 200. As a result, pushing fails with
UnexpectedHTTPStatusCode("pushing blob (POST)", 200, ...), pulling fails
to parse the manifest and "tart login docker.io" accepts any credentials,
because ping() never gets an authentication challenge.
Send the API requests for docker.io to registry-1.docker.io instead, while
still using docker.io for the pushed image names and for the credentials
lookup, so that credentials saved with "tart login docker.io" keep working.
Fixes#1275
* Match docker.io case insensitively
* Recognize Docker Hub with an explicit port
* Parse the registry host once and keep it normalized
Using the host exactly as specified for naming and credentials lookup
changed the behavior for other registries too. For example,
"127.0.0.1:05000" used to find credentials stored for "127.0.0.1:5000",
but didn't anymore.
Parse the URL once instead, take the normalized host and port from it
like before, and only replace the URL's host with registry-1.docker.io
for Docker Hub.
