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You don’t have to download the internet upon each build.

First, in a corporate environment it’s common to run builds backed by artifact servers that’ll cache just about anything.

Second, it’s easy to place files in a Docker build context (that’s just a $25 dollar way of saying “next to the Dockerfile”) that would have been downloaded from the internet, but are stored locally instead. This is easier said than done for some formats. Source tarballs? Easy. Anything Java or Debian that requires a pesky server which works a certain way? You’re going to have to use a caching artifact server.