fix: revert to Rust image

Dockerfile

@@ -1,5 +1,5 @@
-ARG ALPINE_TAG=3.23
-FROM alpine:${ALPINE_TAG} AS builder
+ARG RUST_TAG=1.94.1-alpine3.23
+FROM rust:${RUST_TAG} AS builder
 
 ARG BUILD_BASE_VERSION=0.5-r3
 RUN --mount=type=cache,sharing=locked,target=/var/cache/apk \
@@ -37,9 +37,6 @@ ARG PROTOC_VERSION=31.1-r1
 RUN --mount=type=cache,sharing=locked,target=/var/cache/apk \
     apk add --update protoc=${PROTOC_VERSION}
 
-# TODO: caching for this
-RUN rustup target add x86_64-unknown-linux-musl
-
 ENV CXXFLAGS="-include cstdint"
 RUN \
     # This one would be nice if it worked:
@@ -53,7 +50,7 @@ RUN \
     --mount=type=cache,target=/usr/local/cargo/git/db \
     --mount=type=cache,target=/usr/local/cargo/registry/ \
     cd /root/app && \
-    cargo build --target x86_64-unknown-linux-musl --release && \
+    cargo build --release && \
     cp target/release/fomo-reducer /root/program
 
 FROM scratch AS runner