Dockerfile: Make Docker image building verbose

This ensures each command is printed out before it is executed, which
leads to a much friendlier log for debugging and the like.

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

Author: Lee Jones

Dockerfile

@@ -36,7 +36,7 @@ ENV RC_VERSION 1.1.1
 
 WORKDIR /app
 
-RUN BUILDDEPS="python make git g++" \
+RUN set -x; BUILDDEPS="python make git g++" \
 &&  apt-get update &&  apt-get install -y --no-install-recommends ${BUILDDEPS} \
 &&  curl -fSL "https://releases.rocket.chat/${RC_VERSION}/download" -o rocket.chat.tgz \
 &&  curl -fSL "https://releases.rocket.chat/${RC_VERSION}/asc" -o rocket.chat.tgz.asc \