Document docker upload file path workaround

[bvandermoon]

Description

Saw this error when running upload_maxtext_docker_image CLOUD_IMAGE_NAME=${CLOUD_IMAGE_NAME?}:

ERROR: Found symbolic links with absolute paths in the build context:
./maxtext_venv/bin/python
Docker cannot follow absolute paths outside of the build context, which can cause 'failed to compute cache key' errors.
Please remove these links or convert them to relative paths before building the Docker image.
Alternatively, run the command again from a clean, empty directory to bypass your local file state entirely

Fixed it by adding maxtext_venv/bin/python to a new .dockerignore file. Thanks @SurbhiJainUSC for the fix

Tests

upload_maxtext_docker_image CLOUD_IMAGE_NAME=${CLOUD_IMAGE_NAME?}

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