Docker compose error: nginx: [emerg] no host in upstream ":9000"

Question

When I run my Docker-based application with docker-deploy docker-compose-deploy.yml up I have this error in my terminal:

no host in upstream ":9000" in /etc/nginx/co
proxy_1  | nginx: [emerg] no host in upstream ":9000" in /etc/nginx/conf.d/default.conf:

My docker-compose-deploy.yml contains:

FROM  nginxinc/nginx-unprivileged:1-alpine
LABEL maintainer="londonappdeveloper.com"

COPY ./default.conf.tpl /etc/nginx/default.conf.tpl
COPY ./uwsgi_params /etc/nginx/uwsgi_params
COPY ./run.sh /run.sh

ENV LISTEN_PORT=8000
ENV AP_HOST=app
ENV APP_PORT=9000

USER root

RUN mkdir -p /vol/static && \
    chmod 755 /vol/static && \
    touch /etc/nginx/conf.d/default.conf && \
    chown nginx:nginx /etc/nginx/conf.d/default.conf && \
    chmod +x /run.sh

VOLUME /vol/static

USER nginx

CMD ["/run.sh"]

and in my default.conf.tpl:

server {
    listen ${LISTEN_PORT};

    location /static {
        alias /vol/static;
    }

    location / {
        uwsgi_pass              ${APP_HOST}:${APP_PORT};
        include                 /etc/nginx/uwsgi_params;
        client_max_body_size    10M;
    }
}