Update docker.io/rust Docker tag to v1.78 (#13)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| docker.io/rust | minor | `1.77` -> `1.78` |

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Reviewed-on: #13
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.woodpecker.yml

@@ -1,5 +1,5 @@
 variables:
-  - &rust_image "docker.io/rust:1.77"
+  - &rust_image "docker.io/rust:1.78.0"
   - &slow_check_paths
     - path:
         # rust source code