Feature Request: Base Image Lockfile

Gradle has dependency version lockfiles.

I can declare my [dependency versions](https://docs.gradle.org/current/userguide/dependency_versions.html) as `latest.release` or `[3,4[`(any version 3.x, but not 4.x).

This offers flexibility at the cost of build reproducibility.

I can restore reproducibility by turning on [dependency locking](https://docs.gradle.org/current/userguide/dependency_locking.html).

Once I've done all of that I get the best of both worlds.

Anytime I want to float to the latest version bounded by all of my dependency version ranges, I run `./gradlew dependencies --write-locks`.

It is my desire to have a similar capability with jib base image declarations.

Whenever I run some command like `--write-locks` the sha of the latest version of my base image is discovered and then written to some kind of lock file. That sha is then used until such time as I run `--write-locks` again.

Of course all of this can be done manually. I've just gotten so used to the convenience afforded by the combination of gradle's version range declarations and its lockfile capability.