ci: expand dependabot config to cover uv and docker (#179)

## Summary

Expands the Dependabot configuration beyond the existing
`github-actions` updates to cover the rest of the repo's dependency
surface.

- **`uv` ecosystem** — tracks Python dependencies in `pyproject.toml` /
`uv.lock`.
- **`docker` ecosystem** — tracks the base image in
`.devcontainer/Dockerfile`.
- **Conventional-commit prefixes** — all ecosystems use `prefix:
"chore"` + `include: "scope"`, producing messages like `chore(deps):
...` / `chore(deps-dev): ...` so generated PRs pass the PR-title check.
- **Grouping** — `substrait-protobuf`, `substrait-antlr`, and
`substrait-extensions` are version-pinned together, so they're grouped
into a single PR rather than three conflicting ones.

Note: `pixi.lock` is intentionally not covered, as Dependabot has no
native support for pixi/conda.

🤖 Generated with AI

Author: nielspardon

.github/dependabot.yml

@@ -4,3 +4,28 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    commit-message:
+      prefix: "chore"
+      include: "scope"
+
+  - package-ecosystem: "uv"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "chore"
+      include: "scope"
+    groups:
+      substrait:
+        patterns:
+          - "substrait-protobuf"
+          - "substrait-antlr"
+          - "substrait-extensions"
+
+  - package-ecosystem: "docker"
+    directory: "/.devcontainer"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "chore"
+      include: "scope"