build: replace uses of archive.CanonicalTarNameForPath

As it's just an alias for filepath.IsAbs. Also added a normalize step in
TrimBuildFilesFromExcludes, so that callers are not _required_ to first
normalize the path.

We are considering deprecating and/or removing this function in the archive
package, so removing it in the cli code helps transitioning if we decide to
deprecate and/or remove it.

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

cli/command/image/build/dockerignore.go

@@ -32,6 +32,9 @@ func TrimBuildFilesFromExcludes(excludes []string, dockerfile string, dockerfile
 	if keep, _ := fileutils.Matches(".dockerignore", excludes); keep {
 		excludes = append(excludes, "!.dockerignore")
 	}
+
+	// canonicalize dockerfile name to be platform-independent.
+	dockerfile = filepath.ToSlash(dockerfile)
 	if keep, _ := fileutils.Matches(dockerfile, excludes); keep && !dockerfileFromStdin {
 		excludes = append(excludes, "!"+dockerfile)
 	}