build.gradle

// additional required configuration to hook into the build script
android {
	signingConfigs {
		release {
			// specify signing properties on the command line
			if (project.hasProperty('keyStore')) {
				println 'Generating a signed package.'
				storeFile file(keyStore)
				storePassword storePass
				keyAlias alias
				keyPassword keyPass
			}
	        }
	}

	buildTypes {
		release {
			if (project.hasProperty('keyStore')) {
				signingConfig signingConfigs.release
			}
		}
	}
}

release.sh

#!/bin/bash

if [[ $# -ne 2 ]]; then
	echo "Usage: $0 keystore keyalias"
	exit 1
fi

if [[ ! -f $1 ]]; then
	echo "$1 doesn't exist or isn't a keystore"
	exit 1
fi

read -s -p "Keystore Password: " STORE_PASS
echo
read -s -p "Key Password: " KEY_PASS
echo

export ORG_GRADLE_PROJECT_keyStore="$1"
export ORG_GRADLE_PROJECT_storePass="$STORE_PASS"
export ORG_GRADLE_PROJECT_alias="$2"
export ORG_GRADLE_PROJECT_keyPass="$KEY_PASS"

./gradlew signingReport

read -p "Is this correct? [y/n] " -n 1 -r
echo
if [[ $REPLY =~ ^[Yy]$ ]]; then
	./gradlew build
fi