v1.1.0 — pinned NetXMS version (6.2.3)

Pinned NetXMS version

Both images now install a fixed NetXMS version instead of whatever packages.netxms.org
serves at build time. A plain rebuild no longer turns into an unintended upgrade — which
for the server meant a crash loop, because a newer netxmsd refuses an existing database
(Your database has format version X, but server is compiled for version Y). Upgrading is
now a deliberate act that can include nxdbmgr upgrade.

• Default: NetXMS 6.2.3 on Ubuntu 24.04, for both server and agent images.
• Override per build: docker build --build-arg NETXMS_VERSION=6.2.4 -f Dockerfile.server .
• An explicit Debian revision is accepted (6.2.0-2) for releases not shipped as -1.
• NETXMS_VERSION=latest restores the previous unpinned behaviour.
• With Compose, set NETXMS_VERSION in .env before docker compose build.
• All netxms-* packages (server/agent/base/dbdrv-sqlite3) are pinned and apt-mark holded
  together, so a pin older than the current release resolves correctly.

Images:

ghcr.io/oriolrius/docker-netxms-server:v1.1.0
ghcr.io/oriolrius/docker-netxms-agent:v1.1.0

Based on #1 by @sabatligats.