ci(devcontainer): use prebuilt image instead of broken Dockerfile build

The devcontainer image build fails on main: the Dockerfile does
`COPY requirements.txt`, but that file was removed when the repo moved
its dependencies to pyproject.toml, so there is nothing to copy. Switch
devcontainer.json to pull the upstream mcr.microsoft.com/devcontainers/python
image and install pre-commit/ruff/uv in postCreateCommand. Fully within
.json so it stays keeper-safe; the now-unused Dockerfile can be dropped in
a follow-up once the keeper accepts the Dockerfile filename (#228).

Author: priya-sundaram-dev

.devcontainer/devcontainer.json

@@ -1,21 +1,25 @@
 {
 	"name": "Python 3",
-	"build": {
-		"dockerfile": "Dockerfile",
-		"context": "..",
-		"args": {
-			// This repo tracks the latest-and-greatest CPython on the newest
-			// stable Debian. The devcontainer base images are published per
-			// CPython minor version on Debian 13 "Trixie" (3.11-trixie ...
-			// 3.14-trixie); there is no floating `3-trixie` tag, so bump this
-			// to the newest available when a new stable CPython ships.
-			// NOTE: these images do not publish free-threaded (`t`) variants,
-			// so 3.14t cannot be selected via the tag alone.
-			"VARIANT": "3.14-trixie"
-		}
-	},
 
-	"postCreateCommand": "zsh .devcontainer/post_install",
+	// Use a prebuilt dev container image instead of building from a local
+	// Dockerfile. The repo migrated its dependencies to pyproject.toml, so the
+	// old Dockerfile's `COPY requirements.txt` step no longer had a file to copy
+	// and the image build failed. The upstream images already ship Python + a
+	// full toolchain, so pulling one is both faster and less to maintain.
+	//
+	// This repo tracks the latest-and-greatest CPython on the newest stable
+	// Debian. Images are published per CPython minor version on Debian 13
+	// "Trixie" (3.11-trixie ... 3.14-trixie); bump this to the newest available
+	// when a new stable CPython ships.
+	// NOTE: these images do not publish free-threaded (`t`) variants, so 3.14t
+	// cannot be selected via the tag alone -- but the repo's `.python-version`
+	// pins 3.14t, and `uv run`/`uv sync` in the container honor it, so uv gives
+	// contributors free-threaded 3.14t regardless of the base tag.
+	"image": "mcr.microsoft.com/devcontainers/python:3.14-trixie",
+
+	// Install the tools post_install and CI expect (pre-commit + ruff), plus uv
+	// for the free-threaded workflow above, then run the existing setup script.
+	"postCreateCommand": "pipx install pre-commit ruff uv && zsh .devcontainer/post_install",
 
 	// Configure tool-specific properties.
 	"customizations": {