fix: consider tar (pax or ustar (+ extended pax)) as archives

[akatsukilelouch]

previously it just considered compressed data as archives but docs specify that plain uncompressed tar is also supported but it was not.

per documentation:

REQUEST BODY SCHEMA: application/octet-stream

A tar archive compressed with one of the following algorithms: identity (no compression), gzip, bzip2, xz.
string <binary>

when using docker API the http context containing was never extracted if it was plain tar. some libraries and clients send plain tar, but that fails when using buildkit.

[tonistiigi]

This is not a valid way to detect uncompresed tar. First is just ././ and "ustar" does not appear in the beginning of a file.

[akatsukilelouch]

I am sorry, I've forgotten how tar stores files. I had a specific use-case and the pax header file was the first the block and it had this quite weird pax header file path, didn't recheck the details of pax format, thought it is the standard.

Yeah...

[akatsukilelouch]

Maybe it's better to try parse the tar file and, if failed, keep the buffer as is? Or it's better to ditch uncompressed file support completely? Waiting for a decision. I'll implement the tar parsing later, because I need it temporarily. Please close as not planned if you're ditching tar support in the API. Thank you.