Added node 21.4 variant of 5.5.0

Author: Dan Lynn

Dockerfile

@@ -1,4 +1,4 @@
-FROM node:20.10.0-bullseye
+FROM node:21.4.0-bullseye
 MAINTAINER Dan Lynn <[email protected]>
 
 # ember server on port 4200

README.md

@@ -11,7 +11,7 @@ This image contains everything you need to have a working development environmen
 ### Supported tags and respective `Dockerfile` links
 
 + [`5.5.0`,`5.5.0-node_20.10`,`latest` (5.5.0/Dockerfile)](https://github.com/danlynn/ember-cli/blob/5.5.0/Dockerfile)
-+ [`5.5.0-node_21.2` (5.5.0-node_21.2/Dockerfile)](https://github.com/danlynn/ember-cli/blob/5.5.0-node_21.2/Dockerfile)
++ [`5.5.0-node_21.4` (5.5.0-node_21.4/Dockerfile)](https://github.com/danlynn/ember-cli/blob/5.5.0-node_21.4/Dockerfile)
 + [`5.4.1`,`5.4.1-node_20.10` (5.4.1/Dockerfile)](https://github.com/danlynn/ember-cli/blob/5.4.1/Dockerfile)
 + [`5.4.1-node_21.2` (5.4.1-node_21.2/Dockerfile)](https://github.com/danlynn/ember-cli/blob/5.4.1-node_21.2/Dockerfile)
 + [`5.4.0`,`5.4.0-node_20.9` (5.4.0/Dockerfile)](https://github.com/danlynn/ember-cli/blob/5.4.0/Dockerfile)