docker with rails dependencies

Why is that docker image doesn't capture all the dependencies of a new rails app? (it saved most gems except some built with native extension ones)

For example, I use Dockerfile to create a rails app and then build the image. Then I create a container from that image, inside the container I create a 2nd app, the bundle install command will pass faster than the first app since it doesn't have to re-install most gems, but then it stalls on the built-with-native-extension ones. Finally I create a 3rd app in the same container, this time it passes quickly without any stalling.

What's going on with the second app, why hasn't the first app took care of all dependencies already?

submitted by /u/usedocker
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Source: Reddit