docker-compose.yml: drop version '3'

Drop the obsolete version: '3' property, it is no longer needed since
late 2020 (Docker Compose 1.27+) and is only informative:

https://nickjanetakis.com/blog/docker-tip-51-which-docker-compose-api-version-should-you-use
https://github.com/compose-spec/compose-spec/blob/master/04-version-and-name.md

Signed-off-by: Tim Orling <tim.orling@konsulko.com>

docker-compose.yml

@@ -1,4 +1,3 @@
-version: '3'
 services:
   layersdb:
     image: mariadb:lts