Add Keycloak local testing environment (#604)

* Adding one more example - keycloak - alongside with dex IDP.

* don't expose keycloak and proxy ports to the host

* specify email-domain list option in documentation

* get rid of nginx and socat to simplify the example as per https://github.com/oauth2-proxy/oauth2-proxy/pull/604#issuecomment-640054390

* get rid of the scripts - use static file for keycloak startup

* changelog entry

* Update CHANGELOG.md

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>