fix(core): Add kwargs to image build

[Tranquility2]

Fix: #706, https://github.com/testcontainers/testcontainers-python/pull/614

Now when using kwargs in the Image API, the params are passed correctly into the build

with DockerImage(path=dir, tag="test", buildargs={"MY_ARG": "some_arg"}) as image:

Added relevant test + updated docstring to better reflect the usage

[codecov[bot]]

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[black-snow]

Can we perhaps add a hint to the docstring that kwargs will be passed down to docker-py? That would pretty much solve https://github.com/testcontainers/testcontainers-python/pull/614 for me.

[Tranquility2]

@black-snow Updated docstring as you suggested.