Upgrade to AlmaLinux 10 and fix WebSocket proxying

Base image moves from AlmaLinux 9 to 10, which forces two related changes:
EPEL repo URL bumps to the el10 release RPM, and tini is replaced with
dumb-init as PID 1 since tini is not packaged in EPEL 10. Both provide the
signal forwarding and zombie reaping the wedged-container fix relies on.
Nginx 1.26 in AlmaLinux 10 deprecates the `listen ... http2` parameter, so
the server block now uses the separate `http2 on;` directive.

Also fixes three issues that affected WebSocket apps (socket.io, ws):

- `Connection: upgrade` was hardcoded on every proxied request, including
  ordinary HTTP. Now driven by a `map $http_upgrade $connection_upgrade`
  so only genuine upgrade requests carry it.
- No explicit proxy read/send timeout meant idle WebSockets were cut at
  nginx's 60s default. Set to 600s, which clears any sane heartbeat without
  pinning connection slots (worker_connections is 512, two per client).
- `large_client_header_buffers 2 1k` returned 400 for any single header
  line over 1KB, which session cookies and bearer tokens routinely exceed.
  Raised to the nginx default of 4 8k; buffers are allocated on demand, so
  this only costs memory for requests that need it.

Verified by rendering the generated config and running it under nginx with
a Node backend: WebSocket handshakes return 101 and reach the upstream
'upgrade' event, polling requests arrive with `Connection: close`, and a
3KB cookie returns 200 where the old buffer setting returned 400.

README gains a WebSocket Support section covering the PM2 cluster-mode
trap, the concurrency ceiling, and reconnects on memory restarts.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -1,11 +1,11 @@
-FROM almalinux/9-base
+FROM almalinux/10-base
 ARG NODEVER=20
 
 # Install repos, update, install only needed packages, clean up in one layer
 RUN dnf install -y \
-      https://dl.fedoraproject.org/pub/epel/epel-release-latest-9.noarch.rpm && \
+      https://dl.fedoraproject.org/pub/epel/epel-release-latest-10.noarch.rpm && \
     dnf update -y && \
-    dnf install -y wget procps cronie iproute nginx openssl git microdnf make gcc gcc-c++ tini && \
+    dnf install -y wget procps cronie iproute nginx openssl git microdnf make gcc gcc-c++ dumb-init && \
     dnf group install -y 'Development Tools' && \
     dnf clean all && \
     rm -rf /var/cache/dnf /usr/share/doc /usr/share/man /usr/share/locale/* \