switch dev.Dockerfile to use node14 alpine, changes node-sass version to use node 14 binary

[mattxwang]

No description provided.

[reginawang3495]

Anything I can help with here? Is this something that you were still thinking of getting out?

[mattxwang]

If we figure out #94 and #92, this is probably unneeded; this was just to see if we could resolve an issue Dylon was having running the membership portal.

(the gist of the issue is that node-sass is a native module, and the dockerfile's build process installs node_modules first, then enters the container; so, the native modules aren't built properly for the docker host OS, and things fail.)