Cache layer with docker to speed up the development workflow

[francescotaioli]

Describe your idea/feature/enhancement

[Project: sam init - python 3.8 - image container (not zip file)]
sam build always rebuild the image even if:

• requirements.txt is not changed
• the code is not changed

Proposal

At least in development, we could change to Dockerfile to cache the layers that didn't changed, such as requirments.txt. When someone has a slow connection, it take A LOT of time.

In order to do this, the default Dockerfile should be changed in:
New Dockerfile

FROM public.ecr.aws/lambda/python:3.8 AS build

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

COPY app.py ./
COPY model /opt/ml/model

CMD ["app.lambda_handler"]

Old Dockerfile

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

COPY app.py requirements.txt ./
COPY model /opt/ml/model

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

CMD ["app.lambda_handler"]

Additional Details

[awood45]

This makes sense as a feature request - one reason the build behaves this way is that even if, for example, requirements.txt is unchanged, dependencies may have changed within constraints and a rebuild can be appropriate/intended. However, I can see where options to limit the layers built can come in handy.

[sleepwithcoffee]

I think we can close this ticket as it is addressed under #362