build: make cross-compilation workflow better

Dockerfile

@@ -1,4 +1,4 @@
-FROM oven/bun:1-alpine AS css-builder
+FROM --platform=$BUILDPLATFORM oven/bun:1-alpine AS css-builder
 WORKDIR /temp
 COPY package.json bun.lockb ./
 RUN bun install --frozen-lockfile
@@ -7,23 +7,33 @@ COPY tailwind.config.js .
 COPY src ./src
 RUN bunx tailwindcss -i ./src/style.css -o ./out.css --minify
 
-FROM --platform=$TARGETPLATFORM rust:1.80-alpine AS build
+FROM --platform=$BUILDPLATFORM rust:1.80-alpine AS build
 ARG TARGETPLATFORM
+ENV TARGETPLATFORM=${TARGETPLATFORM}
 
-RUN apk add musl-dev --no-cache
+RUN apk add musl-dev --no-cache && rustup target add x86_64-unknown-linux-musl aarch64-unknown-linux-musl
 
 WORKDIR /usr/src
 RUN USER=root cargo new --bin vesta
 WORKDIR /usr/src/vesta
 COPY Cargo.toml Cargo.lock ./
 COPY src ./src
-RUN cargo build --release
+# Determine the correct target triple based on the platform
+RUN if [ "$TARGETPLATFORM" = "linux/amd64" ]; then \
+    export TARGET=x86_64-unknown-linux-musl; \
+    elif [ "$TARGETPLATFORM" = "linux/arm64" ]; then \
+    export TARGET=aarch64-unknown-linux-musl; \
+    else \
+    echo "Unsupported platform: $TARGETPLATFORM"; \
+    exit 1; \
+    fi && \
+    cargo build --release --target $TARGET
 
 FROM scratch
 
 WORKDIR /app
 COPY ./static static
 COPY --from=css-builder /temp/out.css ./static/style.css
-COPY --from=build /usr/src/vesta/target/release/vesta /app/vesta
+COPY --from=build /usr/src/vesta/target/$TARGET/release/vesta /app/vesta
 
 CMD ["/app/vesta"]