Fixes issue #364

[remie]

Description

As described in #364, it is not possible to run firebase serve --only functions in a dockerized environment because for some reason configstore.get('tokens') does not contain any values, regardless of whether you pass --token or set the FIREBASE_TOKEN environment variable.

What does work, is to actually switch to process.env.FIREBASE_TOKEN if there isn't a set of tokens available in the config store. This will also work with --token.

Scenarios Tested

Create a new Firebase Functions project with firebase-tools, Dockerize the application with firebase serve --only functions --token $FIREBASE_TOKEN as the command, or with FIREBASE_TOKEN=<TOKEN> firebase serve --only functions and watch it run smoothly.

Sample Dockerfile

FROM node:8

ARG FIREBASE_TOKEN
ARG FB_PROJECTID
ENV FIREBASE_TOKEN ${FIREBASE_TOKEN}
ENV FB_PROJECTID ${FB_PROJECTID}

WORKDIR /opt/sample
COPY ./firebase.json firebase.json

WORKDIR /opt/sample/functions
COPY ./functions/package.json package.json
COPY ./functions/package-lock.json package-lock.json
RUN npm install; \
        npm install -g firebase-tools; \
	firebase use $FB_PROJECTID;

COPY ./functions/src src
COPY ./functions/tsconfig.json tsconfig.json
COPY ./functions/tslint.json tslint.json

CMD ["npm", "run", "serve"]