Upgrade devcontainer to Ubuntu 26.04

Now that the image is available at mcr.microsoft.com/devcontainers/base,
switch from the deprecated vscode/devcontainers/base path and resolve the
TODO that was tracking this.

Author: yeikel

.devcontainer/Dockerfile

@@ -1,8 +1,6 @@
-# See here for image contents: https://github.com/microsoft/vscode-dev-containers/tree/v0.245.2/containers/ubuntu/.devcontainer/base.Dockerfile
-
-# [Choice] Ubuntu version (use ubuntu-22.04 or ubuntu-18.04 on local arm64/Apple Silicon): ubuntu-22.04, ubuntu-20.04, ubuntu-18.04
-ARG VARIANT="jammy"
-FROM mcr.microsoft.com/vscode/devcontainers/base:0-${VARIANT}
+# [Choice] Ubuntu version: ubuntu26.04, ubuntu24.04, ubuntu22.04
+ARG VARIANT="ubuntu26.04"
+FROM mcr.microsoft.com/devcontainers/base:${VARIANT}
 
 # [Optional] Uncomment this section to install additional OS packages.
 # RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \

.devcontainer/devcontainer.json

@@ -1,16 +1,12 @@
 // For format details, see https://aka.ms/devcontainer.json. For config options, see the README at:
-// https://github.com/microsoft/vscode-dev-containers/tree/v0.245.2/containers/ubuntu
-// TODO: Upgrade to ubuntu-26.04 once available in mcr.microsoft.com/vscode/devcontainers/base
-// See: https://github.com/devcontainers/images/issues/1858
-// Check availability with: curl -s -o /dev/null -w "%{http_code}" https://mcr.microsoft.com/v2/vscode/devcontainers/base/manifests/0-ubuntu-26.04
+// https://github.com/devcontainers/images/tree/main/src/base-ubuntu
 {
   "name": "Ubuntu",
   "build": {
     "dockerfile": "Dockerfile",
-    // Update 'VARIANT' to pick an Ubuntu version: jammy / ubuntu-22.04, focal / ubuntu-20.04, bionic /ubuntu-18.04
-    // Use ubuntu-22.04 or ubuntu-18.04 on local arm64/Apple Silicon.
+    // Update 'VARIANT' to pick an Ubuntu version: ubuntu26.04, ubuntu24.04, ubuntu22.04
     "args": {
-      "VARIANT": "ubuntu-22.04"
+      "VARIANT": "ubuntu26.04"
     }
   },
   // Use 'forwardPorts' to make a list of ports inside the container available locally.