Dockerfile: Tell tar not to be verbose

There is no need to print out the name of every file as it is
extracted.  This only serves to cloud the log making it unusable.

Signed-off-by: Lee Jones <lee.jones@linaro.org>

Author: Lee Jones

Diff for: Dockerfile

@@ -22,7 +22,7 @@ RUN set -x; BUILDDEPS="python make git g++" \
 &&  curl -fSL "https://releases.rocket.chat/${RC_VERSION}/download" -o rocket.chat.tgz \
 &&  curl -fSL "https://releases.rocket.chat/${RC_VERSION}/asc" -o rocket.chat.tgz.asc \
 &&  gpg --batch --verify rocket.chat.tgz.asc rocket.chat.tgz \
-&&  tar zxvf rocket.chat.tgz \
+&&  tar zxf rocket.chat.tgz \
 &&  rm rocket.chat.tgz rocket.chat.tgz.asc \
 &&  cd bundle/programs/server \
 &&  npm install \