Improve the way app waits for db to be ready

credits : https://gitlab.utc.fr/picasoft/projets/dockerfiles/-/merge_requests/64

app/Dockerfile

@@ -25,6 +25,7 @@ RUN apk add --no-cache \
 	netcat-openbsd \
 	xmlsec-dev \
 	tzdata \
+	postgresql-client \
 	&& rm -rf /tmp/*
 
 # Get Mattermost

app/entrypoint.sh

@@ -74,7 +74,11 @@ if [ "$1" = 'mattermost' ]; then
 
   # Wait another second for the database to be properly started.
   # Necessary to avoid "panic: Failed to open sql connection pq: the database system is starting up"
-  sleep 1
+  until pg_isready -h $DB_HOST -p $DB_PORT_NUMBER
+  do
+	  echo "Database is not ready yet. Waiting 5 seconds."
+	  sleep 5
+  done
 
   echo "Starting mattermost"
 fi