Make CI smarter about when to rebuild the base images #94

Job	Run time
lint	10s
base-images	5m 14s
app-images (nightly, pulp-minimal)	17m 24s
app-images (nightly, pulp)	32m 5s
app-images (nightly, galaxy-minimal)	12m 15s
app-images (nightly, galaxy)	13m 57s
app-images (stable, pulp-minimal)	12m 53s
app-images (stable, pulp)	27m 54s
app-images (stable, galaxy-minimal)	10m 51s
app-images (stable, galaxy)	12m 15s
	2h 24m 58s

Provide feedback