Add input validation to check if image repository name and tag are provided

[cmoulliard]

• Add input validation to check if image repository name and tag are provided
• Otherwise an error will be raised.
• Update the documentation to mention the 2 required fields
• Include part of the pre-req that a backstage container image is required.

Issue: #16

[cmoulliard]

Can you merge it as it LGTM to Chris too ? @vinzscam

[vinzscam]

Can you merge it as it LGTM to Chris too ? @vinzscam

sorry I've been away. LGTM! ❤️