Add workflows for auto-building and releasing the application images.

[hedss]

• Builds on PR for image testing, with branch tags.
• Builds on PR merge with latest and main tags.
• Builds with version publishing to allow versioned tags.

Been meaning to do this for ages, and recent rebuild woes have forced my hand.

[hedss]

Needs secrets adding to GH before merging.

[hedss]

Secrets are now added.