apksigner can be installed on Termux. And ubuntu in Termux.
pkg install apksigner
To sign an Android APK, you need your own Android key store test.jks created before.
Usage:
#From https://stackoverflow.com/a/47647383
apksigner sign --ks test.jks --ks-key-alias test --ks-pass pass:testtest --key-pass pass:testtest upcal.apk