Improve configuration sources

[sagikazarmark]

Is your feature request related to a problem?

When running Dex in a container on any container orchestration platform, some configuration options (listen addresses in particular) should be configurable in the deployment context, while some other options (connectors, clients, etc) are relevant in the applications domain (regardless in what environment it runs in).

Dex has a single configuration file and requires setting all (or most) configuration options in that file.

Describe the solution you'd like to see

Operators should be able to override configuration using environment variables or flags. That way they can set specific options in the deployment context without having to modify the configuration file. For example, in a helm chart typically users can supply their own configuration while address and port settings are configured differently.

Some settings that might need a flag:

• web.http
• web.https (although this is an optional setting)
• telemetry.http
• grpc.addr

Some settings that might need an env var:

• frontend.dir so that a container image can restrict which dir to serve

Describe alternatives you've considered

We could switch to Viper eventually, but for the short term, I don't think that's necessary.

We could also play with yaml merging in the Helm chart, but that's weird and fragile. Furthermore, that doesn't work well with custom provided secrets.