Add latest tag to Beta builds

[willvanwazer]

Now that the IPTVBoss team is recommending¹ switching to the beta, it would be helpful to have a latest tag also for the iptv-boss-beta set of releases.

Footnotes

1. https://discord.com/channels/667770358333702144/1007618077946552411/1307021316087349369 ↩

[github-actions]

@groenator is more or less the only maintainer of this project and works on it in his free time.
Please:

• I am only supporting the docker image build. Any issues with the actual BOSS application please raise it with the developer on Discord.
• Do not ask for updates, be patient
• Read carefully the documentation before raising a new issue. Some issues are already addressed there.
• If you have other questions, ask on Discord
  @groenator usually checks issues at least once a week, if this is a new urgent bug, [revert to an older tagged container image]