Consistently use "COPY .. ./" in Dockerfiles

This avoids potential issues and confusion with cases where a plain "." as
target doesn't work.

Dockerfile

@@ -27,7 +27,7 @@ ARG TOOLCHAIN_ARCHS="i686 x86_64 armv7 aarch64"
 
 # Install the usual $TUPLE-clang binaries
 COPY wrappers/*.sh wrappers/*.c ./wrappers/
-COPY install-wrappers.sh .
+COPY install-wrappers.sh ./
 RUN ./install-wrappers.sh $TOOLCHAIN_PREFIX && \
     rm -rf /build