Cache is missed when building Layer inside a container

[Macok]

Description:

When a Layer is built using --use-container option and any --container-env-var are passed, the cache is always missed.
This was already fixed by @mndeveci here: #2943, but only for Lambda Functions, while the same problem exists for layers.

Steps to reproduce:

Run below command twice to build any Layer (it will work fine in case of Functions) and see that the cache is not valid.
sam build --cached --use-container --container-env-var FOO=BAR

Observed result:

Cache is missed.

Expected result:

Cache should be hit.

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

1. OS: Linux
2. sam --version: SAM CLI, version 1.27.2
3. AWS region: N/A

Why does this happen?

Just like in case of #2943, a dictionary of env variables is being passed to the build function by DefaultBuildStrategy.build_single_layer_definition. Additional LAMBDA_BUILDERS_LOG_LEVEL variable is added to the dictionary, causing cache miss. We need to make a deepcopy of this dictionary, just like in DefaultBuildStrategy.build_single_function_definition.

I'm happy to work on a fix as soon as someone confirms my observations.

[sriram-mv]

@Macok sounds like a very good contribution to take on.

[Macok]

Awesome, I'll submit a PR later this week.

[Macok]

Done, waiting for review: #3139