@VisibleForTesting
  static boolean areProxyPropertiesSet(String protocol) {
    return PROXY_PROPERTIES.stream()
        .anyMatch(property -> System.getProperty(protocol + "." + property) != null);
  }
Check if any proxy system properties are already set for a given protocol. Note, <code>
nonProxyHosts</code> is excluded as it can only be set with <code>http</code>.

@param protocol protocol
jib-maven-plugin/src/main/java/com/google/cloud/tools/jib/maven/MavenSettingsProxyProvider.java