Bump library/rust in /backend in the minor group

Bumps the minor group in /backend with 1 update: library/rust.


Updates `library/rust` from 1.77.2-bullseye to 1.78.0-bullseye

---
updated-dependencies:
- dependency-name: library/rust
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: minor
...

Signed-off-by: dependabot[bot] <[email protected]>

backend/Dockerfile

@@ -1,4 +1,4 @@
-FROM docker.io/library/rust:1.77.2-bullseye AS build
+FROM docker.io/library/rust:1.78.0-bullseye AS build
 
 WORKDIR /app