Supports custom headers when the ui is used as proxy

[Joxit]

Background

Headers can be useful in some cases such as avoid sending credentials when you are on the UI (like #87). Or give to the registry server other properties such as X-Forward-For or Server headers for monitoring.

How to use ?

This is compatible only with static version of the UI and used with REGISTRY_URL variable.
When you want to add a custom header, add to the registry ui a environment variable or entry in /etc/nginx/.env which looks like NGINX_PROXY_HEADER_Custom_Header. All underscores (_) will be replaced by hyphens (-).

Some example of custom headers as variable:

• NGINX_PROXY_HEADER_Authorization for Basic auth credentials
• NGINX_PROXY_HEADER_X_Forwarded_For for identifying the originating IP address of a client

An example is bundled with this PR

Any feedback @marcusblake ?

closes: #87

[Joxit]

I can also add a supports for custom headers from a file, like you did in #87 and this will supersede your PR

[marcusblake]

This is great! I believe adding support for custom headers from files (especially credentials) would be a great feature to add as well.