fix(docker): reap orphaned subprocesses via tini as PID 1 (#15116)

Install tini in the container image and route ENTRYPOINT through
`/usr/bin/tini -g -- /opt/hermes/docker/entrypoint.sh`.

Without a PID-1 init, orphans reparented to hermes (MCP stdio servers,
git, bun, browser daemons) never get waited() on and accumulate as
zombies. Long-running gateway containers eventually exhaust the PID
table and hit "fork: cannot allocate memory".

tini is the standard container init (same pattern Docker's --init flag
and Kubernetes pause container use). It handles SIGCHLD, reaps orphans,
and forwards SIGTERM/SIGINT to the entrypoint so hermes's existing
graceful-shutdown handlers still run. The -g flag sends signals to the
whole process group so `docker stop` cleanly terminates hermes and its
descendants, not just direct children.

Closes #15012.

E2E-verified with a minimal reproducer image: spawning 5 orphans that
reparent to PID 1 leaves 5 zombies without tini and 0 with tini.

tests/tools/test_dockerfile_pid1_reaping.py

@@ -0,0 +1,78 @@
+"""Contract tests for the container Dockerfile.
+
+These tests assert invariants about how the Dockerfile composes its runtime —
+they deliberately avoid snapshotting specific package versions, line numbers,
+or exact flag choices.  What they DO assert is that the Dockerfile maintains
+the properties required for correct production behaviour:
+
+- A PID-1 init (tini) is installed and wraps the entrypoint, so that orphaned
+  subprocesses (MCP stdio servers, git, bun, browser daemons) get reaped
+  instead of accumulating as zombies (#15012).
+- Signal forwarding runs through the init so ``docker stop`` triggers
+  hermes's own graceful-shutdown path.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+DOCKERFILE = REPO_ROOT / "Dockerfile"
+
+
+@pytest.fixture(scope="module")
+def dockerfile_text() -> str:
+    if not DOCKERFILE.exists():
+        pytest.skip("Dockerfile not present in this checkout")
+    return DOCKERFILE.read_text()
+
+
+def test_dockerfile_installs_an_init_for_zombie_reaping(dockerfile_text):
+    """Some init (tini, dumb-init, catatonit) must be installed.
+
+    Without a PID-1 init that handles SIGCHLD, hermes accumulates zombie
+    processes from MCP stdio subprocesses, git operations, browser
+    daemons, etc.  In long-running Docker deployments this eventually
+    exhausts the PID table.
+    """
+    # Accept any of the common reapers.  The contract is behavioural:
+    # something must be installed that reaps orphans.
+    known_inits = ("tini", "dumb-init", "catatonit")
+    installed = any(name in dockerfile_text for name in known_inits)
+    assert installed, (
+        "No PID-1 init detected in Dockerfile (looked for: "
+        f"{', '.join(known_inits)}). Without an init process to reap "
+        "orphaned subprocesses, hermes accumulates zombies in Docker "
+        "deployments. See issue #15012."
+    )
+
+
+def test_dockerfile_entrypoint_routes_through_the_init(dockerfile_text):
+    """The ENTRYPOINT must invoke the init, not the entrypoint script directly.
+
+    Installing tini is only half the fix — the container must actually run
+    with tini as PID 1.  If the ENTRYPOINT executes the shell script
+    directly, the shell becomes PID 1 and will ``exec`` into hermes,
+    which then runs as PID 1 without any zombie reaping.
+    """
+    # Find the last uncommented ENTRYPOINT line — Docker honours the final one.
+    entrypoint_line = None
+    for raw_line in dockerfile_text.splitlines():
+        line = raw_line.strip()
+        if line.startswith("#"):
+            continue
+        if line.startswith("ENTRYPOINT"):
+            entrypoint_line = line
+
+    assert entrypoint_line is not None, "Dockerfile is missing an ENTRYPOINT directive"
+
+    known_inits = ("tini", "dumb-init", "catatonit")
+    routes_through_init = any(name in entrypoint_line for name in known_inits)
+    assert routes_through_init, (
+        f"ENTRYPOINT does not route through an init: {entrypoint_line!r}. "
+        "If tini is only installed but not wired into ENTRYPOINT, hermes "
+        "still runs as PID 1 and zombies will accumulate (#15012)."
+    )