Make CI smarter about when to rebuild the base images #92

Job	Run time
base-images	18m 49s
base-images	1m 18s
lint	5s
lint	5s
app-images (nightly, pulp-minimal)	18m 23s
app-images (nightly, pulp)	32m 50s
app-images (nightly, galaxy-minimal)	12m 23s
app-images (nightly, galaxy)	14m 3s
app-images (stable, pulp-minimal)	12m 15s
app-images (stable, pulp)	27m 56s
app-images (stable, galaxy-minimal)	11m 4s
app-images (stable, galaxy)	12m 21s
app-images	0s
	2h 41m 32s

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