Bump smallvec from 0.6.7 to 0.6.14 in /assets/dockerfiles/base/rust

Bumps [smallvec](https://github.com/servo/rust-smallvec) from 0.6.7 to 0.6.14.
- [Release notes](https://github.com/servo/rust-smallvec/releases)
- [Commits](servo/rust-smallvec@v0.6.7...v0.6.14)

---
updated-dependencies:
- dependency-name: smallvec
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>