Change: Drive image compilation with JITSI_IMAGE_VERSION

[Berbe]

The JITSI_IMAGE_VERSION environment variable, part of env.example, is already made for driving images tag selection at runtime.

This proposal aims at:

• Using JITSI_IMAGE_VERSION environment variable also at build time, making image generation and stack deployment coherent
• Ensuring compilation of all images is done with that same tag

[saghul]

This would break the current workflow of make prepare creating latest images for testing.

[saghul]

The release.sh script would be broken.