Chore: Dependency pinning & Depdendabot introduction (#436)

1. Pins the Github Action workflow hashes under `.github/workflows` to
full-length commit SHAs.
2. Adds a base level `.github/dependabot.yml` config to keep these (and
other go/Docker) deps up to date.

## Motivation and Context
- Github's [security
guide](https://docs.github.com/en/actions/reference/security/secure-use#using-third-party-actions)
recommends using the immutable SHA of an action when consuming third
party actions.
- Depdendabot makes ongoing dep house keeping manageable.

.github/dependabot.yml

@@ -0,0 +1,18 @@
+version: 2
+updates:
+  - package-ecosystem: gomod
+    directory: /
+    schedule:
+      interval: daily
+  - package-ecosystem: github-actions
+    directory: /
+    groups:
+      actions:
+        patterns:
+          - "*"
+    schedule:
+      interval: daily
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+        interval: "daily"