(core): BundlingDockerImage.cp() needs to be explained more in the README

When using lambda.Code.fromAsset and cdk.BundlingDockerImage.fromAsset together, synth fails to find anything in \asset-output

Reproduction Steps

1. Create a Dockerfile that compiles and copies files to an /asset-output directory

FROM python:3.7-slim
COPY . /asset-input
COPY . /asset-output
WORKDIR /asset-input
RUN apt-get update && apt-get -y install curl make automake gcc g++ subversion python3-dev
RUN curl -sSL https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py | python3 -
ENV PATH "/root/.poetry/bin:/opt/venv/bin:${PATH}"
RUN poetry export -f requirements.txt -o requirements.txt
RUN pip3 install -r requirements.txt -t /asset-output

1. Use the following snippet when creating the lambda using cdk:

code: lambda.Code.fromAsset(PROJECT_DIR, {
        bundling: {
          image: cdk.BundlingDockerImage.fromAsset(PROJECT_DIR)
        }
      }),

1. Run tsc && cdk synth -o cdk.out

What did you expect to happen?

Docker should find the compiled assets in /asset-output

What actually happened?

Error: Bundling did not produce any output. Check that content is written to /asset-output.

Environment

• CDK CLI Version : 1.75.0
• Framework Version: 1.75.0
• Node.js Version: v15.3.0
• OS : Mac Catalina 10.15.7
• Language (Version): TypeScript 4.1.2

Other

If I use an implementation of ILocalBundling that is mostly copied from asset-staging.ts but calls both run and cp the synth works but I don’t believe that should be necessary:

class LocalBundling implements ILocalBundling {
  tryBundle(outputDir: string, options: BundlingOptions): boolean {

    let user: string;
    if (options.user) {
      user = options.user;
    } else {
      // Default to current user
      const userInfo = os.userInfo();
      user =
        userInfo.uid !== -1 // uid is -1 on Windows
          ? `${userInfo.uid}:${userInfo.gid}`
          : "1000:1000";
    }

    // Always mount input and output dir
    const volumes = [
      {
        hostPath: PROJECT_DIR, // this.sourcePath
        containerPath: AssetStaging.BUNDLING_INPUT_DIR,
      },
      {
        hostPath: outputDir, // bundleDir
        containerPath: AssetStaging.BUNDLING_OUTPUT_DIR ?? outputDir,
      },
      ...(options.volumes ?? []),
    ];

    options.image.run({
      command: options.command,
      user,
      volumes,
      environment: options.environment,
      workingDirectory:
        options.workingDirectory ?? AssetStaging.BUNDLING_INPUT_DIR,
    });

    options.image.cp(AssetStaging.BUNDLING_OUTPUT_DIR ?? outputDir, outputDir);

    return true;
  }
}

---

This is 🐛 Bug Report

[eladb]

const image = BundlingDockerImage.fromAsset('/path/to/docker');

new lambda.Function(this, 'Fn', {
  code: lambda.Code.fromAsset(image.fetch('/path/in/the/image')),
  runtime: lambda.Runtime.NODEJS_12_X,
  handler: 'index.handler',
});

I think the API for BundlingDockerImage can be improved:

const image = Docker.build('/path/to/docker');
const tmpdir = image.cp('/path/in/the/image');
// alternatively, users can specify the destination for "cp"
image.cp('/path/in/the/image', tmpdir);

And then, we can also add something like:

new lambda.Function(this, 'Fn', {
  code: lambda.Code.fromDockerBuildAsset('/path/in/the/image'),
  runtime: lambda.Runtime.NODEJS_12_X,
  handler: 'index.handler',
});

[eladb]

I also ran into a situation where I just wanted to use some content from the built image as the asset output. I think our APIs can probably offer a better experience for this.

1. In this case the asset input is meaningless.
2. Ideally docker cp will be much faster to extract files from the built image as oppose to running a command inside the image.

@jogold what do you think?