Android Gradle detect is a release build or not in runtime?

I need to create auto-increment version mechanism via Gradle. For this I use this manual and create two files

#Fri May 20 11:04:03 EEST 2016
VERSION_BUILD=3

    def versionPropsFile = file('version.properties')

    if (versionPropsFile.canRead()) {
        def Properties versionProps = new Properties()
        versionProps.load(new FileInputStream(versionPropsFile))
        def versionBuild = versionProps['VERSION_BUILD'].toInteger() + 1
        versionProps['VERSION_BUILD'] = versionBuild.toString()
        versionProps.store(versionPropsFile.newWriter(), null)

        defaultConfig {
            applicationId "test.app.com"
            minSdkVersion 18
            targetSdkVersion 23
            multiDexEnabled true
            versionCode  1
            versionName versionBuild
        }
    } else {
        throw new GradleException("Could not read version.properties!")
    }

The my problem is : VERSION_BUILD increments each time when I build project. Not a Run, just a build. And don't matter I Run project or create a signed APK.

The my goal - increment version only when I generate signed APK. How I can do that?