ci: image parameterization via env variables

[rabi-siddique]

This PR introduces the ability to input a custom image name for E2E testing.

Previous Setup:

• Previously, we used the image ghcr.io/agoric/agoric-3-proposals for E2E testing.
• We could specify the tag for this image during testing in CI via an input.

The Issue:

• Recently, images like ghcr.io/agoric/agoric-sdk:a3p-use-upgrade-next-20240911203722-bc66a5 started being used for E2E testing, in u17.
• To test this image in CI, we manually hardcoded the image name in the docker-compose file (as done in #365). This was not ideal.

The Fix:

• This PR solves the issue by allowing us to input the image name directly to test different images in CI.

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