convert to multistage docker build

[nathandunn]

FROM agrdocker/agr_base_linux_env:latest as build-stage
WORKDIR /workdir/agr_ui
ADD . .
RUN npm install
ARG NODE_ENV=production
ENV NODE_ENV ${NODE_ENV}
ARG RELEASE=0.0.0
ENV RELEASE ${RELEASE}
RUN npm run build
RUN npm test
FROM nginx
WORKDIR /workdir/agr_ui/dist
COPY --from=build-stage /workdir/agr_ui/dist /workdir/agr_ui/dist
COPY --from=build-stage /workdir/agr_ui/nginx.conf /etc/nginx/conf.d/default.conf
EXPOSE 2992

[nathandunn]

https://github.com/alliance-genome/agr_jbrowse_container