Make container error message more helpful so users learn about '--pull'

The container build intentionally does not run apt-get update all the time as it would consume a lot of time. Having the apt cache forcefully update on very container rebuild would also render debcraft unusable when being offline. Instead improve the error message so users know what to do if they encounter apt errors such as e.g.:

E: Failed to fetch http://deb.debian.org/debian/pool/main/d/debputy/dh-debputy_0.1.33_all.deb  404  Not Found [IP: 151.101.214.132 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?