Fix processing of unset build-args during build

This reverts 26103.  26103 was trying to make it so that if someone did:
  docker build --build-arg FOO .
and FOO wasn't set as an env var then it would pick-up FOO from the
Dockerfile's ARG cmd.  However, it went too far and removed the ability
to specify a build arg w/o any value. Meaning it required the --build-arg
param to always be in the form "name=value", and not just "name".

This PR does the right fix - it allows just "name" and it'll grab the value
from the env vars if set. If "name" isn't set in the env then it still needs
to send "name" to the server so that a warning can be printed about an
unused --build-arg. And this is why buildArgs in the options is now a
*string instead of just a string - 'nil' == mentioned but no value.

Closes #29084

Signed-off-by: Doug Davis <dug@us.ibm.com>

builder/dockerfile/builder.go

@@ -125,7 +125,7 @@ func NewBuilder(clientCtx context.Context, config *types.ImageBuildOptions, back
 		config = new(types.ImageBuildOptions)
 	}
 	if config.BuildArgs == nil {
-		config.BuildArgs = make(map[string]string)
+		config.BuildArgs = make(map[string]*string)
 	}
 	ctx, cancel := context.WithCancel(clientCtx)
 	b = &Builder{