ci: add dependabot for crates.io (#17)

## Description
<!-- A summary of what this pull request achieves and a rough list of
changes. -->
## Breaking Changes
<!-- Optional, if there are any breaking changes document them,
including how to migrate older code. -->
## Notes & open questions
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## Change checklist
- [ ] Self-review.
- [ ] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.
- [ ] Tests if relevant.
- [ ] All breaking changes documented.

Author: dignifiedquire

.github/dependabot.yaml

@@ -11,3 +11,20 @@ updates:
           - "*"  # Group all Actions updates into a single larger pull request
     schedule:
       interval: weekly
+      day: "thursday"
+      time: "08:00"
+      timezone: "America/New_York"
+  - package-ecosystem: cargo
+    directory: /
+    schedule:
+      interval: weekly
+      day: "thursday"
+      time: "08:00"
+      timezone: "America/New_York"
+  - package-ecosystem: docker
+    directory: docker
+    schedule:
+      interval: weekly
+      day: "thursday"
+      time: "08:00"
+      timezone: "America/New_York"