fix: use isError package so function works across realms

Author: Planeshifter

lib/node_modules/@stdlib/assert/is-docker/lib/main.js

@@ -22,6 +22,7 @@
 
 var readFile = require( '@stdlib/fs/read-file' ).sync;
 var contains = require( '@stdlib/assert/contains' );
+var isError = require( '@stdlib/assert/is-error' );
 var exists = require( '@stdlib/fs/exists' ).sync;
 
 
@@ -49,7 +50,7 @@ function isDocker() {
 		return true;
 	}
 	file = readFile( '/proc/self/cgroup', OPTS );
-	if ( file instanceof Error ) {
+	if ( isError( file ) ) {
 		return false;
 	}
 	return contains( file, 'docker' );