Improve fontification of docker-files

Improve fontification of docker-files

Add support for fontifying the following grammar-elements:
- image-declarations (image-name, image-tag & image alias)
- strings in more general terms
- string-expansion statements
- treat path-values as strings
- identifiers names for ENV and ARG statements
- argument values for ENV and ARG statements
* lisp/progmodes/dockerfile-ts-mode.el (dockerfile-ts-mode)
(dockerfile-ts-mode--font-lock-settings): Improve fontification.
(Bug#76481)

(cherry picked from commit 2044f6d9592cdec3a916c0d3c0a8a079c479db61)