A lightweight APK signing tool that can be run on Android devices.
It is available as a package in Termux which can be installed with pkg install apksigner.
Run as apksigner [-p password] keystore input-apk output-apk. This will use the specified keystore (or creating one if necessary) to create a signed and zipaligned output file.
Apache License, Version 2.0. Based on zip-signer by Ken Ellinwood.