Skip unnecessary braces for consistency

Dockerfile

@@ -55,8 +55,8 @@ ENV TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
 
 RUN cd mingw-w64/mingw-w64-headers && \
     for arch in $TOOLCHAIN_ARCHS; do \
-        mkdir build-${arch} && cd build-${arch} && \
-        ../configure --host=${arch}-w64-mingw32 --prefix=$TOOLCHAIN_PREFIX/${arch}-w64-mingw32 \
+        mkdir build-$arch && cd build-$arch && \
+        ../configure --host=$arch-w64-mingw32 --prefix=$TOOLCHAIN_PREFIX/$arch-w64-mingw32 \
             --enable-secure-api --with-default-win32-winnt=0x600 && \
         make install && \
         cd .. || exit 1; \