Describe the bug

When using kwargs in the Image API, the params are not passed into the build

To Reproduce

For example:

with DockerImage(path="./", tag="new:test", buildargs={"PIP_EXTRA_INDEX_URL": PIP_EXTRA_INDEX_URL}) as image:

The PIP_EXTRA_INDEX_URL fails to register in the docker image build process:

E               docker.errors.BuildError: The command '/bin/sh -c pip3 install --no-cache-dir -r requirements.txt --extra-index-url ${PIP_EXTRA_INDEX_URL}' returned a non-zero code: 2