Fix the replication test.

cmd/postgres_exporter/tests/docker-postgres-replication/Dockerfile.p2

@@ -2,6 +2,7 @@ FROM postgres:{{VERSION}}
 MAINTAINER Daniel Dent (https://www.danieldent.com)
 ENV PG_MAX_WAL_SENDERS 8
 ENV PG_WAL_KEEP_SEGMENTS 8
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y inetutils-ping
 COPY setup-replication.sh /docker-entrypoint-initdb.d/
 COPY docker-entrypoint.sh /docker-entrypoint.sh
 RUN chmod +x /docker-entrypoint-initdb.d/setup-replication.sh /docker-entrypoint.sh

cmd/postgres_exporter/tests/docker-postgres-replication/docker-entrypoint.sh

@@ -36,7 +36,7 @@ if [ "$1" = 'postgres' ]; then
 	    if [ "x$REPLICATE_FROM" == "x" ]; then
 		eval "gosu postgres initdb $POSTGRES_INITDB_ARGS"
 	    else
-            	until ping -c 1 -W 1 ${REPLICATE_FROM}
+            	until /bin/ping -c 1 -W 1 ${REPLICATE_FROM}
             	do
                 	echo "Waiting for master to ping..."
                 	sleep 1s