docker/add-user.sh: Support getting email and password from env variables

This patch extends docker/add-user.sh to support getting the email and
password from environment variables.

That way, docker/add-user.sh can be used in scripts.

https://github.com/albertito/chasquid/pull/43

Amended-by: Alberto Bertogli <albertito@blitiri.com.ar>
  Minor edits to the commit message.

docker/add-user.sh

@@ -7,16 +7,19 @@
 
 set -e
 
-read -r -p "Email (full user@domain format): " EMAIL
+if test -z "${EMAIL:-}"; then
+        read -r -p "Email (full user@domain format): " EMAIL
+fi
 
 if ! echo "${EMAIL}" | grep -q @; then
 	echo "Error: email should have '@'."
 	exit 1
 fi
 
-
-read -r -p "Password: " -s PASSWORD
-echo
+if test -z "${PASSWORD:-}"; then
+        read -r -p "Password: " -s PASSWORD
+        echo
+fi
 
 DOMAIN=$(echo echo "${EMAIL}" | cut -d '@' -f 2)