Fix dockerfile array parsing

rustup/Dockerfile

@@ -30,6 +30,7 @@ RUN curl https://sh.rustup.rs -sSf | \
 ENV PATH=/root/.cargo/bin:$PATH
 
 ARG EXTRA=""
-RUN for name in "$EXTRA"; do rustup install $name; done
+RUN declare -a ARR=($EXTRA); for name in $ARR; do rustup install $name; done
+
 ARG TARGETS=""
-RUN for name in "$TARGETS"; do rustup target add $name; done
+RUN declare -a ARR=($TARGETS); for name in $ARR; do rustup target add $name; done