client: remove getDockerOS utility in favor of "Ostype" header

This utility was added in 83b5729f64 to
replace httputils.ParseServerHeader, which was added to print a warning
on Windows in 126529c6d0. At the time, the
only available option to detect the daemon's OS was to parse the `Server`
header, which contained the version of Docker as well as the OS.

However, 7199522ea2 introduced an `OSType`
("Ostype") header that's included on all responses, and a later commit
e9dac5ef5e changed that to also be included
when producing an error for unsupported API versions.

Note that the casing in the midddleware was changed from `OSType` to
`Ostype` (normalized form) in 76a5ca1d4d,
but headers are case-insensitive, and `header.Get()` should handle either
case in the response.

In short; every API response contains an "Ostype" header, which already
contains the OS ("windows" or "linux") that doesn't require any parsing,
so let's put that header to use.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/utils.go

@@ -8,12 +8,9 @@ import (
 
 	cerrdefs "github.com/containerd/errdefs"
 	"github.com/docker/docker/api/types/filters"
-	"github.com/docker/docker/internal/lazyregexp"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
-var headerRegexp = lazyregexp.New(`\ADocker/.+\s\((.+)\)\z`)
-
 type emptyIDError string
 
 func (e emptyIDError) InvalidParameter() {}
@@ -31,16 +28,6 @@ func trimID(objType, id string) (string, error) {
 	return id, nil
 }
 
-// getDockerOS returns the operating system based on the server header from the daemon.
-func getDockerOS(serverHeader string) string {
-	var osType string
-	matches := headerRegexp.FindStringSubmatch(serverHeader)
-	if len(matches) > 0 {
-		osType = matches[1]
-	}
-	return osType
-}
-
 // getFiltersQuery returns a url query with "filters" query term, based on the
 // filters provided.
 func getFiltersQuery(f filters.Args) (url.Values, error) {