Tiny image import fix

[darkowlzz]

When an image is very small, filesystem creation using mkfs fails due to
not enough space. This adds a minimum base image size that's used when
the image is too small.

Also, fixes the /etc/resolv.conf symlink error when there's no /etc
directory in the image filesystem. /etc dir is created as part of the
import step if it doesn't exists.

Adds an e2e test for this fix using hello-world docker container image
which is tiny and lacks /etc dir.

Fixes #402

[bbros-dev]

From your comment in #402

I'm not sure about a solution for this. Maybe just log a warning when resolv.conf setup fails?

Can you document the choice made, either in a FAQ type section hinting at the cause of certain errors or in the kernel build documentation?

[darkowlzz]

Can you document the choice made, either in a FAQ type section hinting at the cause of certain errors or in the kernel build documentation?

This will not be a problem anymore after this is merged. This is following the firecracker networking option Kernel Boot Param . Ignite image import was failing to create a symlink from /etc/resolv.conf to /proc/net/pnp because images like hello-world and busybox don't have /etc directory at root directory. With this fix, the import step will create /etc if it doesn't exists.

[bbros-dev]

Testing one of them should be enough

Apologies for not being clearer. I didn't suggest testing both.

Just the record and by way of explanation: From (bitter) experience I always take hello world examples with a shovel of salt - it tends to indicate real usage isn't as trivial as they are trying to make out - so they use 'hello world'. If something real was as simple they would have made/used the real test/demo case.

[darkowlzz]

@bbros-dev As per my understanding, this issue is about two fixes, small size and missing /etc dir. Busybox is about ~1.2 MB, which is more than what we need to test the fix. hello-world image is ~1.8 kB, which fits the need for this small size test. Dockerfile source for both busybox and hello-world are not very different.
If you can recommend any other real image that's as small as hello-world (requirement: image size * 3 < 500 kB) happy to use that.

[bbros-dev]

No worries.

[stealthybox]

Regardless of the lack of a known example for a vm userspace < 0.5MB, this fix is very reasonable.
Thanks all for taking the time to report, discuss, and implement it.