Bump library/rust in /.devcontainer in the minor group

Bumps the minor group in /.devcontainer 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] <support@github.com>

.devcontainer/Dockerfile.backend

@@ -1,4 +1,4 @@
-FROM docker.io/library/rust:1.77.2-bullseye
+FROM docker.io/library/rust:1.78.0-bullseye
 
 RUN rustup component add rustfmt clippy