Function createDelta

Given a source and a destination image, invokes /images/delta and returns a promise to a readable stream for following progress. Can also be called with a callback as the second argument instead, similar to how Dockerode methods support both a callback and async interface.

Callers can extract the delta image ID by parsing the stream like so:

const stream = await docker.createDelta({ src, dest });
const deltaId = await new Promise<string>((resolve, reject) => {
  let imageId: string | undefined = null;
  function onFinish(err) {
    if (err != null) {
      return reject(err);
    }
    if (imageId == null) {
      return reject(new Error('failed to parse delta image ID!'));
    }
    resolve(imageId);
  }
  docker.modem.followProgress(stream, onFinish, (e: any) => {
    const match = /^Created delta: (sha256:\w+)$/.exec(e.status);
    if (match && imageId == null) {
      imageId = match[1];
    }
  });
});
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Deltas are currently only available with balenaEngine, but this method makes no effort to determine whether that's the case.