[lyrixx]

Refs #430, alternative to #449 and #463.

The build cache depends on the permissions of the build context files. A checkout made with a 002 umask gets 664 files instead of the 644 ones of the CI: its first cache miss was the COPY php/mods-available/ of php-base, so the PHP, nginx and Node packages were reinstalled.

The build context files are now copied as late as possible, so a miss only rebuilds cheap steps (COPY, phpenmod...). The worker stage no longer gets mods-available/, it enabled none of them.

#463 avoids every miss with a stage normalizing the permissions, but when frontend and builder are built in parallel, like castor docker:build does, builder loses its cache after php-base and reinstalls Node.

Fresh builder, cache pushed from a 644 checkout, frontend and builder built in parallel and loaded, local registry, median of 3 runs:

	Checkout	Duration	Steps rebuilt (frontend / builder)
main	644 (like the CI)	21s	0 / 0
main	664	83s	12 / 11
#463	664	94s	1 / 9
this PR	664	30s	9 / 3