fix(authn): handle the case where zot with openid runs behind a proxy (#1675)

added a new config option under 'http' called externalURL which is used
by openid/oauth2 clients to redirect back to zot

Signed-off-by: Petu Eusebiu <peusebiu@cisco.com>

pkg/api/config/config.go

@@ -84,6 +84,7 @@ type RatelimitConfig struct {
 //nolint:maligned
 type HTTPConfig struct {
 	Address       string
+	ExternalURL   string `mapstructure:",omitempty"`
 	Port          string
 	AllowOrigin   string // comma separated
 	TLS           *TLSConfig