add labels to Dockerfile

[chris48s]

Closes #7838

I've added 2 labels here.

• One is fly.version. I think this is what flyctl shows when you run flyctl image show 🤞 but this is mainly based on looking at their own docker images e.g: https://github.com/fly-apps/postgres-standalone/blob/ddbaaacf1d946dcd68ab0d532995906d13e3df6f/Dockerfile#L20 rather than documentation - we'll have to see
• The other is version, which just seems like something sensible to set for wider consumers.

[shields-ci]

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[calebcartwright]

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