Wrong base64 encoding #1145

Hi!

Some parts of this project, specifically the ones that encode the X-Registry-Auth header, use base64Encode to encode the JSON object with the username/password etc. However, that's not an url-safe way, and docker-cli encodes it in a slightly different way: moby/moby#33434

So, if your password/username contains special characters that get encoded to something that contains e.g. a slash, docker-java will still insert a slash into the header whereas an underscore should be used. This caused me a lot of headaches when trying to set up testcontainers with a private registry...

Anyway, I think the only reasonable solution is to replace the way docker-java encodes the header. Same follows for the token (see https://docs.docker.com/registry/spec/auth/jwt/, search for base64).