Wrong permissions in volume in Docker container

I run Docker 1.8.1 in OSX 10.11 via an local docker-machine VM.

I have the following docker-compose.yml:

web:
    build: docker/web
    ports:
        - 80:80
        - 8080:8080
    volumes:
        - $PWD/cms:/srv/cms

My Dockerfile looks like this:

FROM alpine

# install nginx and php
RUN apk add --update \
    nginx \
    php \
    php-fpm \
    php-pdo \
    php-json \
    php-openssl \
    php-mysql \
    php-pdo_mysql \
    php-mcrypt \
    php-ctype \
    php-zlib \
    supervisor \
    wget \
    curl \
    && rm -rf /var/cache/apk/*

RUN mkdir -p /etc/nginx && \
    mkdir -p /etc/nginx/sites-enabled && \
    mkdir -p /var/run/php-fpm && \
    mkdir -p /var/log/supervisor && \
    mkdir -p /srv/cms

RUN rm /etc/nginx/nginx.conf
ADD nginx.conf /etc/nginx/nginx.conf
ADD thunder.conf /etc/nginx/sites-enabled/thunder.conf

ADD nginx-supervisor.ini /etc/supervisor.d/nginx-supervisor.ini

WORKDIR "/srv/cms"
VOLUME "/srv/cms"

EXPOSE 80
EXPOSE 8080
EXPOSE 22

CMD ["/usr/bin/supervisord"]

When I run everything with docker-compose up everything works fine, my volumes are mounted at the correct place.

But the permissions in the mounted folder /srv/cms look wrong. The user is "1000" and the group is "50" in the container. The webserver could not create any files in this folder, because it runs with the user "root".