Cannot push to android emulator because of Read only file system

For those coming here in 2020:

In my case the steps in the answer didn't work as the version 30 API didn't support a writable system.

I had to use the version 28 API version.

Try this:

$emulator -avd Nexus_5X_API_26_x86 -writable-system 
$adb root
$adb remount
$adb push hosts /system/etc/hosts