build: replace uses of archive.CanonicalTarNameForPath

This function is an alias for filepath.IsAbs and we are considering deprecating
and/or removing this function in the archive package, so removing its uses
helps transitioning if we decide to deprecate and/or remove it.

[docker/cli@fb0788f] also added a normalize step in TrimBuildFilesFromExcludes,
so that callers are not _required_ to first normalize the path, so the
normalizeation may be redunant, but keeping the normalization as this variable
is used further down the code.

[docker/cli@fb0788f]: docker/cli@fb0788f

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

pkg/compose/build_classic.go

@@ -124,7 +124,7 @@ func (s *composeService) doBuildClassic(ctx context.Context, project *types.Proj
 		}
 
 		// And canonicalize dockerfile name to a platform-independent one
-		relDockerfile = archive.CanonicalTarNameForPath(relDockerfile)
+		relDockerfile = filepath.ToSlash(relDockerfile)
 
 		excludes = build.TrimBuildFilesFromExcludes(excludes, relDockerfile, false)
 		buildCtx, err = archive.TarWithOptions(contextDir, &archive.TarOptions{