fix(memcached): use multi-stage build to resolve permission denied on mkdir/chmod

[Copilot]

The official memcached base image sets a non-root USER, so any RUN instruction in a derived image fails when touching system directories like /usr/local/lib/.

Changes

• Multi-stage build: Added a debian:13-slim (Trixie, matching the memcached:1.6.42-trixie runtime) build stage that runs as root to create /usr/local/lib/memcached and fetch routelib.lua via ADD --checksum
• Runtime stage: Replaced the failing RUN mkdir/chmod + ADD with COPY --from=build --chown=65534:65534 — no root operations needed in the final stage

# Build (runs as root)
FROM debian:13-slim@sha256:... AS build
RUN mkdir -p /usr/local/lib/memcached && chmod 755 /usr/local/lib/memcached
ADD --chmod=644 --checksum=sha256:... https://.../routelib.lua /usr/local/lib/memcached/routelib.lua

# Run (non-root memcache user from base image)
FROM memcached:1.6.42-trixie@sha256:...
COPY --from=build --chown=65534:65534 /usr/local/lib/memcached /usr/local/lib/memcached

[Copilot]

Pull request overview

This PR updates the memcached Dockerfile to prevent a build failure (chmod: Operation not permitted) by ensuring /usr/local/lib/memcached is created with the intended permissions before a remote ADD places routelib.lua into it.

Changes:

• Create /usr/local/lib/memcached explicitly via RUN mkdir -p ... && chmod 755 ... prior to the ADD.
• Remove the post-ADD RUN chmod 755 /usr/local/lib/memcached step, since the directory is now prepared ahead of time.

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