Use Docker instead of Vagrant for generate images

Fixes #2

qemu/test.config.go

@@ -4,7 +4,7 @@
 
 package qemukernel
 
-const testConfigVmlinuz = "../tools/qemu-debian-img/vmlinuz-bionic"
-const testConfigInitrd = "../tools/qemu-debian-img/initrd-bionic"
-const testConfigRootfs = "../tools/qemu-debian-img/bionic.img"
-const testConfigSampleKo = "../tools/qemu-debian-img/sample.ko"
+const testConfigVmlinuz = "../tools/qemu-debian-img/ubuntu1804.vmlinuz"
+const testConfigInitrd = "../tools/qemu-debian-img/ubuntu1804.initrd"
+const testConfigRootfs = "../tools/qemu-debian-img/ubuntu1804.img"
+const testConfigSampleKo = "../tools/qemu-debian-img/ubuntu1804.ko"