test_docker: set --shm-size=2g

Without --ipc=host (which was dropped in the previous commit) the
container falls back to Docker's default 64 MB /dev/shm. That is
well-known to cause probabilistic chromium crashes under load
because the renderer/browser shared-memory regions live there.

Reproduced this locally: the unified-flag run without --shm-size
hit 7 SIGSEGV crashes across the suite (Errors: 5, Flakes: 2 over
1688 tests). The crash signature is uniform: kernel-originated
NULL deref ("Received signal 11 SI_KERNEL000000000000").

playwright-browsers is presumably less affected because its tests
don't put the same shm pressure on chrome-headless-shell, but we
shouldn't rely on that. 2 GiB matches what most playwright
distributions ship with.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yury-s

.github/workflows/test_docker.yml

@@ -44,6 +44,7 @@ jobs:
               --platform linux/$ARCH \
               --user=pwuser \
               --workdir /home/pwuser \
+              --shm-size=2g \
               -e CI \
               -e PW_MAX_RETRIES \
               -d -t \