bump rust to 1.32 and alpine to 3.9 for dockerfile freshness

[wk]

Since the rust language is a relatively fast moving project, keeping the artifact build process up with rust's latest is likely to be beneficial and prevent new commits which make use of new language features from breaking the build.

Outright relying on rust:latest for the build, however, could break the build process due to deprecation, or introduce bugs that affect only the docker artifact.

Thus, a compromise in the form of a lightly-tested manual bump to the current latest (1.32.0) instead.

[partim]

We actually have an officially supported minimal Rust version (currently 1.30) and there is a Travis build on that. However, in 1.32 Rust switched to the system allocator for smaller binary sizes, so this is a good idea, anyway.

Which also reminds me that I wanted to do an allocator shootout for Routinator ...

[wk]

Didn't realize how much of a difference that makes! Last build on 1.30.1:
-rwxr-xr-x 1 root root 10036688 Feb 1 12:54 /usr/local/bin/routinator
Test build on 1.32.0:
-rwxr-xr-x 1 root root 7119488 Feb 3 11:57 /usr/local/bin/routinator

Very cool! Thanks for the heads up!