Expose docker build context digest that is used to determine whether to do docker build or not.

[efrench-novi]

Hello!

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Issue details

Currently, the docker Image resource will only build if the build context changes, which is a nice feature that allows skipping unnecessary builds. However, we are currently tagging our images with the git SHA, which forces a new build and image deployment on every commit. This results in deploying all image based resources (Service, Batch jobs, lambdas, etc), even though it is unnecessary.

We would like to be able to tag the image with the value of the build context digest so image dependent resources are only updated when a new image is built (and no other resource changes are made). This will allow for well behaved projects that only build and deploy images when necessary, and only deploy image based resources when the image itself changes.

Affected area/feature

[mjeffryes]

That sounds reasonable; I'll add this to our backlog.