[IMP] Odoo 15.0-17.0: improve deprecated keywords

[kobros-tech]

MAINTAINER keyword became deprecated and it is better to use LABEL keyword followed by key=value. ENV keyword is now using "=" to assign environment variables to their values like: ENV key=value. It is better to define the dockerfile in the first line by this comment: # syntax=docker/dockerfile:1

[kobros-tech]

Why not LABEL maintainer?

If it is necessary I will change, but I also added a definition comment.