Layer caching and build speed

Put the things that change last, last

The builder reuses a cached layer until one instruction changes; from that point everything after it rebuilds. So the order of your instructions is the single biggest factor in build time.

The standard pattern is to copy only the dependency manifest, install dependencies, and then copy the source. A code change then invalidates only the final layers, and the dependency install — the slow part — is reused.

Copying the whole project before installing defeats this entirely: every one-character change reinstalls everything.