Fix Dockerfile to support MSRV 1.81

Fix the Dockerfile such that it's based on a rust image that meets the
minimum supported rust version (MSRV) defined in cargo (1.81).

In a prior commit the MSRV was updated in cargo but not in the
Dockerfile. We should add a check to ensure these two versions remain
synchronized. One way to do this is with a step in the build that builds
the Dockerfile into an image and is required to pass before code can be
committed to the main branch.

Refs: d2cbc3e
Signed-off-by: Ben Webb <[email protected]>

vtn.Dockerfile

@@ -1,4 +1,4 @@
-FROM rust:1.80 as base
+FROM rust:1.81 as base
 RUN apt-get update && apt-get install -y --no-install-recommends curl && apt-get clean
 
 FROM base as builder