Add SKIP_PULL variable for conftest.py

[basfroman]

• apply for running built image during subtensor e2e tests

@MichaelTrestman pls add the variable SKIP_PULL to the docs with the next description or something similar:

Controls whether the Docker image used for end-to-end tests should be pulled from a remote container registry (e.g. GitHub Container Registry) before the test run.
	•	When SKIP_PULL=0 (default), the test framework runs docker pull to ensure the latest version of the image is used.
	•	When SKIP_PULL=1, the pull step is skipped. The tests will only run if the required Docker image is already available locally.
This is useful in CI pipelines where a custom image is built and should not be overwritten by a remote version.