Fix Dockerfile linter warnings

[dfunkt]

• they seem to have started appearing with buildx v0.16.0

Alpine warnings:

13 warnings found (use --debug to expand):
 - FromPlatformFlagConstDisallowed: FROM --platform flag should not use constant value "linux/amd64" (line 37)
 - FromPlatformFlagConstDisallowed: FROM --platform flag should not use constant value "linux/amd64" (line 38)
 - RedundantTargetPlatform: Setting platform to predefined $TARGETPLATFORM in FROM is redundant as this is the default behavior (line 133)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 35)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 36)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 42)
 - FromPlatformFlagConstDisallowed: FROM --platform flag should not use constant value "linux/amd64" (line 29)
 - FromPlatformFlagConstDisallowed: FROM --platform flag should not use constant value "linux/amd64" (line 36)
 - FromPlatformFlagConstDisallowed: FROM --platform flag should not use constant value "linux/amd64" (line 42)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 29)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 34)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 37)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 38)

Debian warnings:

5 warnings found (use --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 29)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 38)
 - FromPlatformFlagConstDisallowed: FROM --platform flag should not use constant value "linux/amd64" (line 29)
 - FromPlatformFlagConstDisallowed: FROM --platform flag should not use constant value "linux/amd64" (line 34)
 - RedundantTargetPlatform: Setting platform to predefined $TARGETPLATFORM in FROM is redundant as this is the default behavior (line 171)

For FromPlatformFlagConstDisallowed I decided to use the 3rd variant from here:
https://github.com/moby/buildkit/blob/master/frontend/dockerfile/linter/docs/FromPlatformFlagConstDisallowed.md#description

[BlackDex]

Thanks for the PR.

I'm fine with the as to AS change, but why add _linux_base to everything?
We do not build anything else, so that seem to be useless and cluttering the code i think. It does not have any value, and i also do not see this mentioned on the link you are revering too.

[dfunkt]

That was needed to fix the FromPlatformFlagConstDisallowed warning.
"Stage name should include the platform, OS, or architecture name to indicate that it only contains platform-specific instructions."
I used the OS variant in order to fix the warnings.

[dfunkt]

I added a way to skip the lint check for FromPlatformFlagConstDisallowed and RedundantTargetPlatform in the new revision.
Only other change that I left in is the as to AS one.