fix(docker): prevent pnpm TTY prompt when BuildKit cache has stale node_modules

pnpm 11 aborts with ERR_PNPM_ABORTED_REMOVE_MODULES_DIR_NO_TTY when it
detects an existing node_modules directory and there is no TTY to confirm
purging it. This happens in Portainer when BuildKit layer cache reuses a
prior deps stage containing node_modules.

- .npmrc: set confirmModulesPurge=false (skip prompt unconditionally)
- Dockerfile deps stage: set CI=true (belt-and-suspenders for pnpm CI mode)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.npmrc

@@ -3,3 +3,4 @@ onlyBuiltDependencies[]=core-js
 onlyBuiltDependencies[]=prisma
 onlyBuiltDependencies[]=sharp
 onlyBuiltDependencies[]=unrs-resolver
+confirmModulesPurge=false