[v0.6 backport] contenthash: ignore system and security xattrs in calculation

[thaJeztah]

backport for the v0.6 / docker-19.03 branch

• contenthash: ignore system and security xattrs in calculation #1393 contenthash: ignore system and security xattrs in calculation
  • fixes COPY cache not re-used depending on SELinux environment #1330 COPY cache not re-used depending on SELinux environment
  • addresses Buildkit: "docker build --cache-from" doesn't use cache from local tagged images (multi-stage builds) moby#39003 (comment)
• contenthash: allow security.capability in cache checksum #1526 contenthash: allow security.capability in cache checksum

[thaJeztah]

@tonistiigi @AkihiroSuda ptal

[tonistiigi]

if I remember correctly it was concluded that this was wrong and security.capability should not be excluded.

[thaJeztah]

good point; I'd have to check; do you know if that was addressed on master?

[tonistiigi]

@thaJeztah I don't think so.

[mttradebyte]

@thaJeztah Do you know if anyone is working on the changes that @tonistiigi mentioned? Clamoring for this change at this point. I'd make them myself and PR but I'm sure it'd be quicker if a member of the project could make the changes and pull them in.

[tonistiigi]

@mtanatwine #1526

[mttradebyte]

Looks like #1526 was merged, so can that get merged into this backport so we can get it in v19?

[thaJeztah]

Cherry-picked #1526. PTAL