fix(core): Add kwargs to image build (#708)

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

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

Added relevant test + updated docstring to better reflect the usage