How to prevent docker from exiting after postgres start

I'm trying to build a customized postgres replication cluster in docker. I'm using postgres:9.6-alpine as my base image. Here is my Dockerfiles which I'm using to build the slave container.

// Dockerfile
FROM postgres:9.6-alpine

ENV GOSU_VERSION 1.10
RUN set -ex; \
    \
    apk add --no-cache --virtual .gosu-deps \
        dpkg \
        gnupg \
        openssl \
    ; \
    \
    dpkgArch="$(dpkg --print-architecture | awk -F- '{ print $NF }')"; \
    wget -O /usr/local/bin/gosu "https://github.com/tianon/gosu/releases/download/$GOSU_VERSION/gosu-$dpkgArch"; \
    wget -O /usr/local/bin/gosu.asc "https://github.com/tianon/gosu/releases/download/$GOSU_VERSION/gosu-$dpkgArch.asc"; \
    \
# verify the signature
    export GNUPGHOME="$(mktemp -d)"; \
    gpg --keyserver ha.pool.sks-keyservers.net --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4; \
    gpg --batch --verify /usr/local/bin/gosu.asc /usr/local/bin/gosu; \
    rm -r "$GNUPGHOME" /usr/local/bin/gosu.asc; \
    \
    chmod +x /usr/local/bin/gosu; \
# verify that the binary works
    gosu nobody true; \
    \
    apk del .gosu-deps

RUN apk add --update iputils
RUN apk add --update htop

# COPY ./setup-slave.sh /docker-entrypoint-initdb.d
COPY ./docker-entrypoint.sh /docker-entrypoint.sh

RUN chmod +x /docker-entrypoint.sh

CMD ["gosu", "postgres", "pg_ctl", "-D/var/lib/postgresql/data", "start"]

And this is my docker-entrypoint.sh:

#!/bin/bash
if [ ! -s "$PGDATA/PG_VERSION" ]; then
echo '*:*:*:myuser:123456' > ~/.pgpass

chmod 0600 ~/.pgpass

until ping -c 1 -W 1 pg_master
    do
        echo "Waiting for master to ping..."
        sleep 1s
done
until pg_basebackup -h pg_master -D ${PGDATA} -U arioo -vP -W
    do
        echo "Waiting for master to connect..."
        sleep 1s
done

echo "host replication all 0.0.0.0/0 md5" >> "$PGDATA/pg_hba.conf"

set -e  
cat >> ${PGDATA}/postgresql.conf <<EOF

wal_level = hot_standby
max_wal_senders = 8
wal_keep_segments = 32
hot_standby = off
EOF

cat > ${PGDATA}/recovery.conf <<EOF
standby_mode = on
primary_conninfo = 'host=pg_master port=5432 user=rep password=123456'
trigger_file = '/tmp/touch_me_to_promote_to_me_master'
EOF
chown postgres. ${PGDATA} -R
chmod 700 ${PGDATA} -R
fi
exec "$@"

The whole thing works just fine except for one thing. Docker immediately exits the slave container upon start. From my understanding pg_ctl start should start the postgres in foreground. But why is docker exiting still?

I use docker-compose to run the cluster.

I even tried putting an endless loop at end of the docker-entrypoint.sh like the code below and it's still exiting no matter what I do:

while true; do
    sleep 1s
done