`expo-updates` uses hard-coded localhost

🐛 Bug Report

Summary of Issue (just a few sentences)

When trying to use REACT_NATIVE_PACKAGER_HOSTNAME to specify where the packager is located, it doesn’t work for building a release app on Android and iOS.

This prevents the builds from completing.

Environment – output of expo diagnostics & the platform(s) you’re targeting

Android (docker container running the Android build)

  Expo CLI 3.21.5 environment info:
      OS: Linux 5.6 Ubuntu 18.04.3 LTS (Bionic Beaver)
      Shell: 4.4.20 - /bin/bash
      Node: 12.18.0 - /usr/local/bin/node
      Yarn: 1.22.4 - /usr/bin/yarn
      npm: 6.14.4 - /usr/local/bin/npm
      expo: ^37.0.0 => 37.0.12
      react: 16.9.0 => 16.9.0
      react-dom: 16.9.0 => 16.9.0
      react-native: 0.61.4 => 0.61.4
      react-native-web: ^0.11.7 => 0.11.7

Reproducible Demo

localhost:8081 is hard-coded:

Expected Behavior vs Actual Behavior

In both files, REACT_NATIVE_PACKAGER_HOSTNAME should used, but using localhost:8081 as a fallback.

I’m trying to work around this by rewriting /etc/hosts in my Docker container from its entrypoint or merging the containers.

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