mcjoin v2.10

• Project CI moved from Travis-CI to GitHub
• Container support: docker pull ghcr.io/troglobit/mcjoin:latest
• Minor bug fixes, found by Coverity Scan
• Fix #18: optional trailing group port arguments ip4/ip6, by Adam Young
• Fix #19: setsid() problem causing mcjoin to exit when daemonized,
  regression introduced in v2.7. Found and fixed by Foster Snowhill
• Fix #23: fix uptime display in old output mode, by Foster Snowhill
• Document alternative use-case, by Torstein Eide