chore: specify fixed image variant in Dockerfile and remove variant argument from devcontainer configuration

.devcontainer/Dockerfile

@@ -1,5 +1,4 @@
-ARG VARIANT=24-bookworm
-FROM mcr.microsoft.com/devcontainers/javascript-node:${VARIANT}
+FROM mcr.microsoft.com/devcontainers/javascript-node:24-bookworm@sha256:21b17a0c1fd000eff7280c9eea21f04779d1825531b144e400e5d028a0d967b1
 
 ARG PNPM_VERSION=11.5.0
 ARG NX_VERSION=22.7.4

.devcontainer/devcontainer.json

@@ -7,7 +7,6 @@
     "dockerfile": "Dockerfile",
     "context": ".",
     "args": {
-      "VARIANT": "24-bookworm",
       "PNPM_VERSION": "11.5.0",
       "NX_VERSION": "22.7.4"
     }