code: protected void decrementAmiSlaveProvision(DockerSlaveTemplate container) {
          synchronized (provisionedImages) {
              int currentProvisioning = 0;
              if (provisionedImages.containsKey(container)) {
                  currentProvisioning = provisionedImages.get(container);
              }
              provisionedImages.put(container, Math.max(currentProvisioning - 1, 0));
          }
      }
docs: Decrease the count of slaves being "provisioned".