[Etherpad] Fix bad environment check at startup

pica-etherpad/entrypoint.sh

 #!/usr/bin/env bash
 set -e
 
-if [ -z "{ETHERPAD_ADMIN_PASSWORD}" ]; then
+if [ -z "${ETHERPAD_ADMIN_PASSWORD}" ]; then
 	echo >&2 'Error : missing required ${ETHERPAD_ADMIN_PASSWORD} environment variable, exiting.'
 	exit 1
 fi
-if [ -z "{ETHERPAD_DB_NAME}" ]; then
+if [ -z "${ETHERPAD_DB_NAME}" ]; then
 	echo >&2 'Error : missing required ${ETHERPAD_DB_NAME} environment variable, exiting.'
 	exit 1
 fi
-if [ -z "{ETHERPAD_DB_USER}" ]; then
+if [ -z "${ETHERPAD_DB_USER}" ]; then
 	echo >&2 'Error : missing required ${ETHERPAD_DB_USER} environment variable, exiting.'
 	exit 1
 fi
-if [ -z "{ETHERPAD_DB_PASSWORD}" ]; then
+if [ -z "${ETHERPAD_DB_PASSWORD}" ]; then
 	echo >&2 'Error : missing required ${ETHERPAD_DB_PASSWORD} environment variable, exiting.'
 	exit 1
 fi
-if [ -z "{ETHERPAD_DB_HOST}" ]; then
+if [ -z "${ETHERPAD_DB_HOST}" ]; then
 	echo >&2 'Error : missing required ${ETHERPAD_DB_HOST} environment variable, exiting.'
 	exit 1
 fi
 
-while ! mysqladmin ping -u${ETHERPAD_DB_USER} -p${ETHERPAD_DB_PASSWORD} -h"$ETHERPAD_DB_HOST"; do
+while ! mysqladmin ping -u${ETHERPAD_DB_USER} -p${ETHERPAD_DB_PASSWORD} -h"${ETHERPAD_DB_HOST}"; do
     sleep 1
 done