fix: two required fixes to make the alpine example work

[monperrus]

congrats on the great repo

[iximiuz]

Hi @monperrus, thanks! Could you elaborate on your set of changes? The Alpine example works fine for me even without this PR being merged. Maybe you could provide the description of the error you're facing in your environment?

[monperrus]

Here is the explanation, on my machine:

• loop devices are read-only, which break image creation, unless docker is run with --privileged
• when you run the alpine.img inside kvm, it does not boot, it is blocked at the initramfs stage, because the root file system is not recognized with mount -t auto. adding rootfstype=ext3 to the kernel command line fixes this.

[monperrus]

Thanks!