perf(build): keep test suites out of the image and cache CI builds durably

Three build-time improvements, all measured on this host.

The build context no longer ships the test suites. `next build` type-checks
the whole project, so `e2e/` and `integration/` in the context made the image
build fail on a type error in test code and pulled @playwright/test, vitest,
msw, testcontainers and drizzle-kit into the image. The CI runs those suites
on the host. Type-check time in the build drops from 2.2s to 1.3s, and a
contract test now fails if application code ever imports from those folders.

The CI now exports the build cache to a directory outside the runner
workspace: /var/cache/epicnext-cms-buildcache for the release and a separate
map for branch preflights, so a branch can never feed the release cache. The
workspace is wiped after every job and `docker builder prune` runs after
every release, so the next build started with an empty cache and downloaded
all 493 packages again (measured: reused 0, downloaded 493). With the
durable export the frozen dependency layer is reused (measured: reused 493,
downloaded 0) even after a full prune. A cache directory that is not
writable only produces a warning; the release is never blocked.

Investigated the remaining ~30s of Next.js build tracing: excluding the
dev-only packages with `outputFileTracingExcludes` made it slower (35s vs
30s), because the traced output was already minimal (5 packages). The cost
is Next's own page-config collection and file tracing over 329 routes, not
the dependency set, so no configuration was kept.

.dockerignore

@@ -26,6 +26,13 @@ bun.lockb
 public/nitro-assets
 public/swf
 
+# Testbronnen draaien op de host (pnpm test:ui / test:integration / test:e2e),
+# de image heeft ze niet nodig. In de image zouden ze alleen de typecheck van
+# `next build` meebepalen en de image van @playwright/test, vitest, msw,
+# testcontainers en drizzle-kit afhankelijk maken.
+e2e/
+integration/
+
 # Databasebackups (~232MB, dagelijkse rotatie): horen niet in de buildcontext.
 # Ze maakten elke context groot en invalideerden bij elke rotatie de
 # `COPY . .`-laag, waardoor de Next-build volledig opnieuw draaide.