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It's a mistake to assume that just because you're using docker, you now shouldn't bother understanding how (debian's) packaging works.

You're usually still installing packages and working with a debian/ubuntu/whatever distribution, except they're in a container now.

Docker is an additional abstraction layer one should understand, not a replacement for an existing one.

If you are used to dealing with debian packaging and running stuff directly on your server, using docker can feel like taking a step forward, but also two backwards. A few things are better, but a bunch are worse.