public void addLabel(String key, String value) {
      if (labels == null) {
        labels = new HashMap<>();
      }
      labels.put(key, value);
    }
Add a label to the container configuration.

@param key the label name to add
@param value the value to be associated with the label
jib-core/src/main/java/com/google/cloud/tools/jib/configuration/ContainerConfiguration.java