Added dockerfiles form trzeci/emscripten-docker

[trzecieu]

As a continuation from: emscripten-core/emscripten#8742

This PR provides adopted Dockerfile from https://github.com/trzecieu/emscripten-docker/blob/master/docker/trzeci/emscripten-fastcomp/Dockerfile which supports builds:

• fastcomp
• upstream

Features:

• An image can be build with fastcomp and upstream version of Emscripten SDK
• It applies some extra size optimizations (that might be removed in sake of clarity / stability)
• It creates conventional 1000:1000 user emscripten
• It creates extra entrypoint that mitigates running as non-root user (Some context: https://trzeci.eu/fixing-permission-of-files-created-from-docker/)
• It has simple-stupid tests performed during building

I understand that some of used solutions might not meet expectations, hence I'm open for suggestions how to improve it.

CC: @kripken @sbc100

[sbc100]

I have lots of small nits but generally I think this I good thing and we should include it here.

If you'd like to address some of these issues after the initial commit I'd be OK with that too.

[sbc100]

This is looking good to me now. @kripken how about you?

Can we add this under https://hub.docker.com/u/emscripten? Perhaps as "emsdk"?

[kripken]

No comments from me, looks reasonable, thanks!

Would it be possible to test this? (as in, test that building this Docker image leads to a working environment?)

Sounds good to add to docker hub. Let me know your usernames there and I'll add you.

[sbc100]

I'm sbc100 on docker hub

[trzecieu]

I'm trzeci on docker hub

[kripken]

Ok, added you two and a team and a repo, hopefully I've done it all correctly, let me know!