Save only when new images were created

[rcowsill]

As described in #81, this action often saves a new cache even though no new images were created. This happens when there were images preloaded and then the action restored a cache successfully.

In that case, checkIfImageHasAdded returns true because existingImages includes the preloaded images as well as the restored ones. That bypasses this check:

action-docker-layer-caching/post.ts

Line 24 in b01c39f

if (await imageDetector.checkIfImageHasAdded(restoredImages)) {

Then getImagesShouldSave returns the list of restored images, so the next check passes because imagesToSave isn't empty.

action-docker-layer-caching/post.ts

Line 30 in b01c39f

if (imagesToSave.length < 1) {

This change ignores restored images when checking if new images were created. When new image(s) are detected they're saved with the restored images in the new cache. This is so that unchanged images aren't lost when other images are rebuilt.

Fixes #81

[sonarcloud]

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[satackey]

@rcowsill Thank you!