Why builds are fast or slow.
WHAT THE CACHE DOES
Reuses the result of an instruction when nothing it depends on has changed.
WHAT INVALIDATES IT
A changed instruction Changed files being copied Any earlier instruction being invalidated
WHY THAT LAST POINT DOMINATES
One early change rebuilds everything after it.
WHAT THAT ARGUES FOR
Ordering instructions from stable to volatile.
WHAT THE CLASSIC MISTAKE IS
Copying the entire source, then installing dependencies.
WHAT THAT CAUSES
Dependencies reinstalled on every source change.
WHAT TO DO INSTEAD
Copy only the dependency manifest, install, then copy the source.
WHAT TO BE CAREFUL WITH
Instructions fetching from the network.
WHY
The cache does not know the remote content changed.
WHAT THAT PRODUCES
Builds using stale packages.
WHAT TO DO ABOUT IT
Pin versions explicitly, rather than relying on cache invalidation.
WHAT A BUILD ARGUMENT PROVIDES
A value supplied at build time.
WHAT TO NEVER PASS AS ONE
A secret.
WHY
It is recorded in the image history and readable by anyone with the image.
WHAT TO USE INSTEAD
Build-time secret mounts, which are not persisted.
WHAT TO DO IN A PIPELINE
Persist the cache between runs, or builds start cold every time.
