Added missing variable KEY to Beszel Template

[ShadowArcanist]

Changes

• Added a new variable KEY

Notes:

• The KEY variable is commented out on the compose so coolify UI is not showing the variable which makes the Beszel Agent not run (Throws environment var KEY not set on terminal).
• This change will make the Beszel Agent run without throwing any error + user can add the value for the variable from the UI.
• The official docs says to enclose the variable within quotes but for me it works without adding the quotes (tried deploying 3 different times and works without any issues)

[vniehues]

I just tested this on my instance and it works great!
I think this should be merged and the docs should reflect the change - currently the docs ask you to add the key to the compose file itself rather than the environment variable section of coolify.