Use io.copy for build context compression

The use of pools here is unnecessary and creates a code dependency on
moby library which will move to internal.

The pool does not provide any value here since it is only done for a
single operation and unlikely performed in alongside other operations
which are re-using the buffers many times between garbage collections.

Signed-off-by: Derek McGowan <derek@mcg.dev>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/image/build/context.go

@@ -17,7 +17,6 @@ import (
 	"github.com/docker/docker/builder/remotecontext/git"
 	"github.com/docker/docker/pkg/archive"
 	"github.com/docker/docker/pkg/ioutils"
-	"github.com/docker/docker/pkg/pools"
 	"github.com/docker/docker/pkg/progress"
 	"github.com/docker/docker/pkg/streamformatter"
 	"github.com/docker/docker/pkg/stringid"
@@ -434,7 +433,7 @@ func Compress(buildCtx io.ReadCloser) (io.ReadCloser, error) {
 		}
 		defer buildCtx.Close()
 
-		if _, err := pools.Copy(compressWriter, buildCtx); err != nil {
+		if _, err := io.Copy(compressWriter, buildCtx); err != nil {
 			pipeWriter.CloseWithError(errors.Wrap(err, "failed to compress context"))
 			compressWriter.Close()
 			return