image: Fix context hash of symlinks in subdirs

Fixes #541.

Evaluates symlinks using the full path to the file, not the incorrect relative
path to the context dir. The provider's current working directory and the
context dir are likely to be different. Regular files are treated similarly:

https://github.com/pulumi/pulumi-docker/blob/8cd8688eaee94c588d65f18862d95a1b0ff3f247/provider/provider.go#L455-L460

provider/provider.go

@@ -442,7 +442,7 @@ func (accumulator *contextHashAccumulator) hashPath(path string, fileMode fs.Fil
 		// a) ignore changes at the symlink target
 		// b) detect if the symlink _itself_ changes
 		// c) avoid a panic on io.Copy if the symlink target is a directory
-		symLinkPath, err := filepath.EvalSymlinks(path)
+		symLinkPath, err := filepath.EvalSymlinks(filepath.Join(accumulator.dockerContextPath, path))
 		if err != nil {
 			return fmt.Errorf("could not evaluate symlink at %s: %w", path, err)
 		}