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There are several things to check: Unused images, Javascript or CSS styles. Importing large libraries like Bootstrap several times can slow things down very much.

In my case it was a SCSS structure that had grown over nearly ten years and caused the Bootstrap styles to be imported over ten times through badly organized imports.

This does not cause problems when precompiling assets in development, because the precompiler caches partial compilation results. However, when executing a Dockerfile the build process starts with an empty cache. The resulting compiled assets will also be very much larger than necessary. 

So: Check your `import` statements.