Here is something new that I learned about how to build and sign an Android Studio gradle project from the command line without editing any files. This makes it very nice for storing your project in version control while keeping your keys and passwords separate from your build.gradle file:

./gradlew assembleRelease \
  -Pandroid.injected.signing.store.file=$KEYFILE \
  -Pandroid.injected.signing.store.password=$STORE_PASSWORD \
  -Pandroid.injected.signing.key.alias=$KEY_ALIAS \
  -Pandroid.injected.signing.key.password=$KEY_PASSWORD

Every suggestion I've seen so far relied on editing build.gradle or doing things manually yourself, but this new solution allows everything to be fully automated. This is how I like to do things.