Improve devcontainer Dockerfile following best practices

- Use apt-get instead of apt for better script compatibility
- Add --no-install-recommends to reduce image size
- Add cleanup to remove apt cache
- Parameterize external tool versions with build args

.devcontainer/devcontainer.json

@@ -3,7 +3,9 @@
   "build": {
     "dockerfile": "Dockerfile",
     "args": {
-      "TZ": "${localEnv:TZ:America/Los_Angeles}"
+      "TZ": "${localEnv:TZ:America/Los_Angeles}",
+      "GIT_DELTA_VERSION": "0.18.2",
+      "ZSH_IN_DOCKER_VERSION": "1.2.0"
     }
   },
   "runArgs": [