Update Dockerfile

Author: mcbarton

ci/docker/Dockerfile

@@ -31,7 +31,7 @@ ENV PATH /opt/cmake/bin:$PATH
 # As with CMake install a later version of Ninja than waht 18.04 has.
 RUN ARCH=$(uname -m) \
   && if [ "$ARCH" = "aarch64"  ]; then SUFFIX=-aarch64; fi \
-  && curl -sSL -o ninja.zip https://github.com/ninja-build/ninja/releases/download/v1.12.1/ninja-linux$(SUFFIX).zip \
+  && curl -sSL -o ninja.zip https://github.com/ninja-build/ninja/releases/download/v1.12.1/ninja-linux${SUFFIX}.zip \
   && unzip ninja.zip \
   && rm *.zip \
   && mv ninja /opt/cmake/bin