refactor labels from Dockerfile into Docker Bake file

minor corrections in Docker Bake file

update README with recent developments

add support for multi-platform image builds

use variable for image created time in Docker Bake file

better variable for multi-tags support in Docker Bake file

refactor Docker build to use `matrix` Bake feature

add ability to add `latest` tags in Docker Bake file

replace brace expansion with for-loop

move assets installation to final image

update upstream OCP source file to version 3.2.0

remove unnecessary `ARG` directive for soundfont

remove unnecessary `ARG` directives for assets

use Git repository in `ADD` directive for OCP

update GeneralUser GS soundfont to latest version