Clean apt cache from image

.gitlab-ci.yml

@@ -83,6 +83,8 @@ build-zynthbox-image-armhf:
     - sudo mount --bind /proc $mountdir/proc
     - sudo mount --bind /sys $mountdir/sys
     - sudo chroot $mountdir apt install zynthbox-z2-setup
+    - sudo chroot $mountdir apt -y clean
+    - sudo chroot $mountdir rm -rf /var/lib/apt/lists/*
     - sudo umount $mountdir/boot/ $mountdir/dev/pts $mountdir/dev/ $mountdir/proc/ $mountdir/sys/
     - sudo umount $mountdir
     # Compress miko image