Make CI smarter about when to rebuild the base images #93

Job	Run time
base-images	5m 56s
lint	5s
app-images (nightly, pulp-minimal)	3m 32s
app-images (nightly, pulp)	3m 34s
app-images (nightly, galaxy-minimal)	3m 38s
app-images (nightly, galaxy)	3m 33s
app-images (stable, pulp-minimal)	3m 32s
app-images (stable, pulp)	3m 34s
app-images (stable, galaxy-minimal)	3m 33s
app-images (stable, galaxy)	3m 33s
	34m 30s

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