build.gradle

apply plugin: 'com.android.application'

// define here sign properties
def sPassword = 'storePassword_here'
def kAlias = 'keyAlias_here'
def kPassword = 'keyPassword_here'

android {
    ...
    signingConfigs {
        release {
            storeFile file("keystore/release.jks")
            storePassword sPassword
            keyAlias kAlias
            keyPassword kPassword
        }
    }
    buildTypes {
        debug {
            signingConfig signingConfigs.release
        }
        release {
            shrinkResources true
            minifyEnabled true
            useProguard true
            signingConfig signingConfigs.release
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    ...
}

...

task generateKeystore() {
    exec {
        workingDir projectDir
        commandLine 'mkdir', '-p', 'keystore'
    }
    exec {
        workingDir projectDir
        commandLine 'rm', '-f', 'keystore/release.jks'
    }
    exec {
        workingDir projectDir
        commandLine 'keytool', '-genkey', '-noprompt', '-keystore', 'keystore/release.jks',
            '-alias', kAlias, '-storepass', sPassword, '-keypass', kPassword, '-dname', 'c=NG',
            '-keyalg', 'RSA', '-keysize', '2048', '-validity', '10000'
    }
}

project.afterEvaluate {
    preBuild.dependsOn generateKeystore
}

https://stackoverflow.com/questions/3997748/how-can-i-create-a-keystore