Update dockerfile to install git for new dependency

https://github.com/jellyfin/jellyfin-web/pull/735

I had to install git to get it to build ✌️

Dockerfile

@@ -3,7 +3,7 @@ ARG FFMPEG_VERSION=latest
 
 FROM node:alpine as web-builder
 ARG JELLYFIN_WEB_VERSION=master
-RUN apk add curl \
+RUN apk add curl git \
  && curl -L https://github.com/jellyfin/jellyfin-web/archive/${JELLYFIN_WEB_VERSION}.tar.gz | tar zxf - \
  && cd jellyfin-web-* \
  && yarn install \