Add docker support for web client (#635)

[Thadah]

Hi, I added some initial docker support because I saw the issue #635 for it and I was also interested in deploying it in my server.

The docker is multistage much like in Fluffychat's case although I decided to stick to the flutter version that was being used for commet instead of latest for stability.

I also added

ARG GIT_HASH=unknown
ARG VERSION_TAG=v0.0.0

To set a specific version and commit with the command:

docker build --build-arg GIT_HASH=$(git rev-parse --short HEAD) --build-arg VERSION_TAG=<version> -t <container_name> .

I have tested the client in localhost and was able to build, deploy and login with my account to my homeserver in https://prisma.moe.

I haven't developed in flutter so the dockerignore might lack some exclusions.

lmk if you feel it needs any changes!

[dot-Justin]

WFM