Detect missing base image (fix #26)

[SarthakBagga]

Fixes #26.

This adds an early check for the base Docker image before running the OS detector or any extensions.

If the image is missing locally, Rocker now:

• Attempts an implicit docker pull with user feedback.
• Fails early with a clear error if the image does not exist.
• Re-raises real Docker errors (network/auth/daemon) instead of hiding them.

This prevents extensions (including nvidia) from crashing on non-existent images and avoids long silent startups.

Testing:

• Missing image → early clear error after failed pull.
• Image not local but exists remotely → pulled and proceeds normally.

[SarthakBagga]

Please let me know if you’d like any changes or improvements to this approach.

[tfoote]

Thanks, I've added a quick unit test to make sure we have some coverage of the logic.