feat: re-exec if rootless using nsenter

[tuananh]

Issue #, if available:
Fixes: #687

Description of changes:

Testing performed:

I setup containerd rootless mode, nsenter and then run soci-snapshoter in there.

Tested index list command

Need to create another PR for snapshotter too if this works

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[tuananh]

@Kern-- is this the correct way to go?

[tuananh]

@Kern-- is this the correct way to go?

[Kern--]

The overall direction looks right. A few comments on dependency updates and license requirements.

I'd also like to see that this works for building indexes too (I think it should, but just to be sure)

[Kern--]

It looks like a lot of this code came directly from nerdctl. That's fine since it's apache 2.0, but we need to add a comment that indicates where this came from and a note about the license/copywrite.

If we can pull this out into another file, it's pretty easy to mark it with a header like:

soci-snapshotter/fs/fs.go

Lines 1 to 31 in 6dcfca7

	/*
	Copyright The Soci Snapshotter Authors.
	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
	You may obtain a copy of the License at
	http://www.apache.org/licenses/LICENSE-2.0
	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	*/
	/*
	Copyright The containerd Authors.
	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
	You may obtain a copy of the License at
	http://www.apache.org/licenses/LICENSE-2.0
	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
	*/

[Kern--]

This doesn't look right. Can we reduce the dependency changes down to just the ones needed for rootless?

It looks like there was a larger update here.

[sondavidb]

Is the intention of this to be able to use rootless containerd with soci? If so, would we also have to make the daemon usable without root? Just curious on the general direction of this PR.

[Kern--]

Sorry for the delayed response. I think we will need to work with the nerdctl maintainers on this.

[Kern--]

I think this creates a circular dependency in nerdctl. Maybe we should see if we can get the rootless util package in a separate module.