fix: redirect issues for hydra integration in domain (not port) based env

[programmador]

Related issue

#67

Proposed changes

1. Replace 'host' header while communicating with kratos to prevent reverse-proxy misbehaviour - see Redirect loop on domain-based configurations #60 . Fixes 302 redirect error with Traefik. Probably with nginx there will be another error: the wrong host will respond to http query. Therefore response will obviously have unexpected content.
2. Make code read Hydra URL from config instead of reading from env var directly. As as side effect it removes trailing slash which fixes 307 redirect error while communicating with Hydra (either in domain-based env or I just have env vars misconfigured - I don't know and it dows not matter because anyway there is no need to read anything from env when we do already have everything in config)

Checklist

• I have read the contributing guidelines
• I have read the security policy
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)

[aeneasr]

Nice, this makes sense!