Improve docker build speed

Dockerfile

@@ -2,26 +2,29 @@ ARG base="alpine:3.17"
 
 FROM ${base} AS bin-builder
 
+# Disable static linking, see: https://users.rust-lang.org/t/sigsegv-with-program-linked-against-openssl-in-an-alpine-container/52172
+# Enable cargo sparse index for faster update times, see: https://blog.rust-lang.org/inside-rust/2023/01/30/cargo-sparse-protocol.html
+ENV RUSTFLAGS='-C target-feature=-crt-static' \
+    CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse
 RUN apk add --no-cache cargo openssl-dev musl-dev
 # This will build all dependencies and store them in docker's cache.
 # This way, it won't be necessary to recompile everything everytime
-# It also disables static linking, see: https://users.rust-lang.org/t/sigsegv-with-program-linked-against-openssl-in-an-alpine-container/52172
 COPY backend/Cargo.toml .
 COPY backend/Cargo.lock .
 RUN echo '[[bin]]' >> Cargo.toml && \
     echo 'name = "cache"' >> Cargo.toml && \
     echo 'path = "cache.rs"' >> Cargo.toml && \
     echo 'fn main() {eprintln!("Caching crates...")}' > cache.rs && \
-    RUSTFLAGS='-C target-feature=-crt-static' cargo build --release 
+    cargo build --release 
 RUN rm cache.rs && \
     rm Cargo.toml
 # Build wimple-wkd
 COPY backend/Cargo.toml .
 COPY backend/src src
-RUN RUSTFLAGS='-C target-feature=-crt-static' cargo build --release
+RUN cargo build --release
 
 
-FROM ${base} AS webpage-builder
+FROM --platform=$BUILDPLATFORM ${base} AS webpage-builder
 
 RUN apk add --no-cache npm
 COPY website .