define ocis_url as a shared option in ocis-pkg/config

proxy/pkg/config/config.go

@@ -105,6 +105,8 @@ type Cache struct {
 
 // Config combines all available configuration parts.
 type Config struct {
+	OcisURL string
+
 	File                  string          `mapstructure:"file"`
 	Log                   Log             `mapstructure:"log"`
 	Debug                 Debug           `mapstructure:"debug"`

proxy/pkg/config/mappings.go

@@ -123,7 +123,7 @@ func structMappings(cfg *Config) []mapping {
 			Destination: &cfg.InsecureBackends,
 		},
 		{
-			EnvVars:     []string{"PROXY_OIDC_ISSUER", "OCIS_URL"}, // PROXY_OIDC_ISSUER takes precedence over OCIS_URL
+			EnvVars:     []string{"PROXY_OIDC_ISSUER"},
 			Destination: &cfg.OIDC.Issuer,
 		},
 		{