fix(ci): use mirrored rust:alpine from private registry

- Change FROM rust:alpine to FROM registry.nxtgauge.com/rust:alpine
- Fixes Docker Hub rate limiting/UNAUTHORIZED errors in Woodpecker builds
- Requires manually pulling and pushing rust:alpine to registry.nxtgauge.com first

Dockerfile.simple

@@ -3,7 +3,7 @@
 
 ARG SERVICE_NAME
 
-FROM rust:alpine AS builder
+FROM registry.nxtgauge.com/rust:alpine AS builder
 ARG SERVICE_NAME
 
 # Install deps