Add memcached-proxy image with pinned digest and embedded routelib

[Copilot]

Memcached Docker image based on memcached:1.6.42-trixie with built-in proxy support, since the official image doesn't ship the proxy route library.

Changes

• Dockerfile: Based on memcached:1.6.42-trixie pinned by digest (sha256:26983a43c...), embeds routelib.lua from memcached/memcached-proxylibs pinned to commit e3c024fb0d
• CI: Added service to docker-compose.ci.yml and Makefile container-structure-test entry
• Tests: Verifies memcached version, proxy support (proxy_config option), and presence of /usr/local/lib/memcached/routelib.lua

Usage

# Standard mode
docker run bdossantos/memcached-proxy

# Proxy mode with embedded routelib
docker run -v /path/to/config.lua:/etc/memcached/proxy.lua:ro \
  bdossantos/memcached-proxy \
  memcached -o proxy_config=/usr/local/lib/memcached/routelib.lua:/etc/memcached/proxy.lua

References

• https://docs.memcached.org/features/proxy/install/
• https://github.com/memcached/memcached-proxylibs