fix: simplify Dockerfile.source and resolve build issues (#231)

- Remove complex dependency caching to fix workspace structure issues
- Remove sccache to eliminate rustc wrapper errors
- Ensure target installation in build step for cross-compilation
- Add debug output and error handling for unsupported platforms
- Use simple COPY . . approach for more reliable builds

Dockerfile.source

@@ -79,13 +79,20 @@ RUN cargo run --bin gproto
 # Build the actual application with optimizations
 RUN case "$TARGETPLATFORM" in \
         "linux/amd64") \
+            echo "🔨 Building for amd64..." && \
+            rustup target add x86_64-unknown-linux-gnu && \
             cargo build --release --target x86_64-unknown-linux-gnu --bin rustfs -j $(nproc) && \
             cp target/x86_64-unknown-linux-gnu/release/rustfs /usr/local/bin/rustfs \
             ;; \
         "linux/arm64") \
+            echo "🔨 Building for arm64..." && \
+            rustup target add aarch64-unknown-linux-gnu && \
             cargo build --release --target aarch64-unknown-linux-gnu --bin rustfs -j $(nproc) && \
             cp target/aarch64-unknown-linux-gnu/release/rustfs /usr/local/bin/rustfs \
             ;; \
+        *) \
+            echo "❌ Unsupported platform: $TARGETPLATFORM" && exit 1 \
+            ;; \
     esac
 
 # Runtime stage - Ubuntu minimal for better compatibility