Running npm run test in the Dockerfile?

Using a builder to generate a smaller docker image, what would be a good way to run npm run test? I seems like running it in the Dockerfile after the build would make sense but maybe I'm missing something

Dockerfile

# Global args to persist through build stages
ARG docker_build_user
ARG docker_build_time
ARG docker_build_head
ARG docker_build_head_short
ARG docker_build_submodules_head

FROM node:8.9.4-alpine as builder

WORKDIR /app

COPY . .

RUN apk add --no-cache bash
RUN apk add --no-cache git
RUN apk add --no-cache make gcc g++ python

RUN npm install

ENV NODE_ENV=production

RUN npm run build

RUN rm -rf node_modules

RUN npm install

FROM node:8.9.4-alpine

# setup build metadata
ARG docker_build_user
ARG docker_build_time
ARG docker_build_head
ARG docker_build_head_short
ARG docker_build_submodules_head

WORKDIR /app

COPY --from=builder /app .

ENV DOCKER_BUILD_USER $docker_build_user
ENV DOCKER_BUILD_TIME $docker_build_time
ENV DOCKER_BUILD_HEAD $docker_build_head
ENV DOCKER_BUILD_HEAD_SHORT $docker_build_head_short
ENV DOCKER_BUILD_SUBMODULES_HEAD $docker_build_submodules_head
ENV DOCKER_BUILD_DESCRIPTION This build was created by $docker_build_user at $docker_build_time from $docker_build_head_short
ENV NODE_ENV=production
ENV ENABLE_LOGGING=true

RUN echo "DESCRIPTION:${DOCKER_BUILD_DESCRIPTION}"

RUN chown -R 999:999 .
USER 999

# expose our service port
EXPOSE 8080

# Default is to run the server (should be able to run worker)
# Set env var in k8s or run : NPM_RUN_TASK (default is serve)
CMD ["/app/startup.sh"]