feat: tests demonstrating copy file interfaces

[rhoban13]

In #676 for copying to/from DockerContainer, it was suggested that we should clarify the interface & usage a bit before proceeding. This PR aims to push that conversation forward with some test cases illustrating proposed usages. Although here I created a Transferable object, in most cases there's no need for the caller to explicitly construct a Transferable, just pass the bytes | Path

Proposed use cases:

1. Copy into a container by passing a Transferrable into the initializer:

DockerContainer(... transferrables=(Transferrable(source, destination_in_container), ...)

2. Copy into the container via the builder pattern, prior to starting the container:

DockerContainer(...)
     .with_copy_into_container(b"some_bytes", destination_in_container)
     .with_copy_into_container(some_path, destination_in_container)

3. Copy into the container at runtime:

with DockerContainer(...) as container:
    container.copy_into_container(b"some_bytes", destination_in_container)
    container.copy_into_container(some_path, destination_in_container)

Note: I've omitted tests where the source or destination is a directory. These should be added after the above use cases are flushed out.

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.
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Files with missing lines	Patch %	Lines
core/testcontainers/core/container.py	94.73%	1 Missing and 1 partial ⚠️

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