feature: Allow specifying Docker build target

[mikkelam]

Description:

I'm trying to use the DockerBuildArgs option to use a specific target in my Dockerfile like so :

...
    Metadata:
      Dockerfile: Dockerfile
      DockerContext: .
      DockerBuildArgs:
        target: prod
...

However it does not work. I'm getting multiple warnings from docker that build args were not consumed

Steps to reproduce:

Dockerfile like so

FROM public.ecr.aws/lambda/python:3.8 as prod

COPY app.py requirements.txt ./

RUN python3.8 -m pip install -r requirements.txt -t .

# Command can be overwritten by providing a different command in the template directly.
CMD ["app.lambda_handler"]

FROM prod as dev
CMD []

along with sam build

Observed result:

`[Warning] One or more build-args [target] were not consumed`

Expected result:

I would be able to use a specific target with sam

Additional environment details (Ex: Windows, Mac, Amazon Linux etc)

1. OS: macos Big Sur
2. sam --version: SAM CLI, version 1.15.0

[mgrandis]

It seems the DockerBuildArgs param is used to pass parameters to the DockerFile at build time.
That's why you get the warning for unused/not consumed parameter.
We are not supporting specifying a target for now but we'll review adding this feature.
This would basically mean allowing a DockerBuildTarget element under Metadata.

Docker python client reference: https://docker-py.readthedocs.io/en/stable/images.html

[mgrandis]

Released in 1.20.0