Use ENV key=value in Dockerfile templates

[StefanScherer]

Hi,
In our docs.docker.com we've removed the examples that used the old syntax ENV key value and replaced it with the better ENV key=value syntax (related docker/docs#11414).
The old style had its problems when using multiple environment variables ENV ONE TWO= THREE=world

Just yesterday I used the Docker extension in preparation for a Docker beginners video course and saw that the Docker extension uses the old style.

This PR changes the templates to the new style and helps users to use the good practice.

[bwateratmsft]

Looks good to me, thanks @StefanScherer!

[thaJeztah]

Thanks! FWIW (for people arriving here); both syntaxes will continue to be supported, but the syntax without = can be somewhat ambiguous, and it's easy to make mistakes with that format, so we're trying to discourage its use. (more background in moby/buildkit#1692 (comment))