Add heredoc support

[href]

Not sure if experimental features are of interest for this project, but I would assume Buildkit's heredoc support is here to stay: https://www.docker.com/blog/introduction-to-heredocs-in-dockerfiles/

This would be especially nice for the bash flavor of the syntax.

[ggari-robotnik]

I made this first try, but I'm far from undestanding how works everything, It still have issues but you can try on my fork https://github.com/ggari-robotnik/Docker.tmbundle

Any help will be appreciated