fix syntax error in Dockerfile

docker/Dockerfile.static

@@ -1,14 +1,12 @@
 # Build stage
 FROM --platform=$BUILDPLATFORM rust:1.75-slim AS builder
 WORKDIR /usr/src/app
-RUN set -x && \
-    apt-get update && \
-    apt-get install -y musl-tools
+RUN apt-get update && apt-get install -y musl-tools
 COPY . .
 
 # Install cross-compilation tools based on target architecture
 ARG TARGETARCH TARGETOS
-RUN case "$TARGETOS/$TARGETARCH" in \
+RUN case "${TARGETOS}/${TARGETARCH}" in \
         "linux/amd64")  TARGET="x86_64-unknown-linux-musl" ;; \
         "linux/arm64")  TARGET="aarch64-unknown-linux-musl" ;; \
         "darwin/amd64") TARGET="x86_64-apple-darwin" ;; \
@@ -19,7 +17,7 @@ RUN case "$TARGETOS/$TARGETARCH" in \
 
 # Build for the target architecture
 ARG TARGETARCH TARGETOS
-RUN case "$TARGETOS/$TARGETARCH" in \
+RUN case "${TARGETOS}/${TARGETARCH}" in \
         "linux/amd64")  cargo build --release --target x86_64-unknown-linux-musl ;; \
         "linux/arm64")  cargo build --release --target aarch64-unknown-linux-musl ;; \
         "darwin/amd64") cargo build --release --target x86_64-apple-darwin ;; \
@@ -30,13 +28,7 @@ RUN case "$TARGETOS/$TARGETARCH" in \
 # Runtime stage
 FROM scratch
 ARG TARGETARCH TARGETOS
-COPY --from=builder /usr/src/app/target/${TARGETOS:+"$(case "$TARGETOS/$TARGETARCH" in \
-        "linux/amd64")  echo "x86_64-unknown-linux-musl" ;; \
-        "linux/arm64")  echo "aarch64-unknown-linux-musl" ;; \
-        "darwin/amd64") echo "x86_64-apple-darwin" ;; \
-        "darwin/arm64") echo "aarch64-apple-darwin" ;; \
-        *)             echo "x86_64-unknown-linux-musl" ;; \
-    esac)"}/release/single-page-web-server-rs /
+COPY --from=builder /usr/src/app/target/${TARGETOS}/${TARGETARCH}/release/single-page-web-server-rs /
 COPY index.html /
 ENTRYPOINT ["/single-page-web-server-rs"]
 CMD ["--addr", "0.0.0.0"]