Don't empty the /tmp directory on docker image build

We should not empty the `/tmp` directory when the docker image is built. The base image `mojdigital/wordpress-base` includes a file called `/tmp/keeptmp` which is used to mitigate a bug whereby `yas3fs` self-destructs by deleting the entire `/tmp` directory. This is a known issue which has been reported to the developers of `yas3fs`: danilop/yas3fs#150

The bug here is that we were emptying the `/tmp` directory at build time, thus disabling the `yas3fs` bug workaround.

Dockerfile

@@ -4,7 +4,7 @@ FROM mojdigital/wordpress-base:latest
 RUN curl -sL https://deb.nodesource.com/setup_8.x | bash - && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y nodejs && \
     apt-get clean && \
-    rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* /init
+    rm -rf /var/lib/apt/lists/* /var/tmp/* /init
 
 ADD . /bedrock