How do I debug an APK that is signed for release?

I know this is old question, but future references. In Android Studio with Gradle:

buildTypes {
    release {
        debuggable true
        minifyEnabled true
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
    }
}

The line debuggable true was the trick for me.

Before Gradle 1.0 it was runProguard instead of minifyEnabled. Look at here.