Nginx Docker Volumes empty - mysql

Question

When using docker-compose, nginx isn't showing files like other images might.
For example with Mysql, the below code will save the data created at /var/lib/mysql to the local machine at ./volumes/db/data
./volumes/db/data:/var/lib/mysql
Another example, with Wordpress, the below code will save the data created at /var/www/html/wp-content/uploads to the local machine at ./volumes/uploads/data
./volumes/uploads/data:/var/www/html/wp-content/uploads
This is not working with nginx though so no matter what I change /some/nginx/path to, it never appears at ./volumes/nginx/data
./volumes/nginx/data:/some/nginx/path
Does nginx work differently in this regard?
Update
Using a named volume with the following configurations solved this problem:
In the services section of the docker-compose file, I changed ./volumes/nginx/data:/some/nginx/path to nginx_data:/some/nginx/path
And then my volumes section reads as follows
volumes:
nginx_data:
driver: local
driver_opts:
o: bind
device: ${PWD}/volumes/nginx/data

Answer 1

There should be no difference, a volume is mounting a local directory to a directory in the container. Either you are not mounting correctly or you are mounting an incorrect path inside the nginx container (one which nginx does not use).
Based on the offical nginx docker image docs on https://docs.docker.com/samples/library/nginx/ you should mount to /usr/share/nginx/html
$ docker run --name some-nginx -v /some/content:/usr/share/nginx/html:ro -d nginx
In additonal I would include full paths in your docker-compose.yaml:
volumes:
- /full_path/volumes/nginx/data:/usr/share/nginx/html
If this is still not working you should exec into the container and confirm that the directory is mounted:
$ docker exec -it <container_name> sh
$ df -h | grep nginx
# write data, confirm you see it on the docker host's directory
$ cd /usr/share/nginx/html
$ touch foo
# on docker host
$ ls /full_path/volumes/nginx/data/foo
If any of this is failing I would look at docker logs to see if there was an issue mounting the directory, perhaps a path or permission issue.
$ docker logs <container_name>
--- UPDATE ---
I ran everything you are using and it just worked:
$ cat Dockerfile
FROM nginx
RUN touch /usr/share/nginx/html/test1234 && ls /usr/share/nginx/html/
$ docker build -t nginx-image-test .; docker run -p 8889:80 --name some-nginx -v /full_path/test:/usr/share/nginx/html:rw -d nginx-image-test; ls ./test;
...
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
33ccbea6c1c1 nginx-image-test "nginx -g 'daemon of…" 4 minutes ago Up 4 minutes 0.0.0.0:8889->80/tcp some-nginx
$ cat test/index.html
hello world
$ curl -i http://localhost:8889
HTTP/1.1 200 OK
Server: nginx/1.15.3
Date: Thu, 06 Sep 2018 15:35:43 GMT
Content-Type: text/html
Content-Length: 12
Last-Modified: Thu, 06 Sep 2018 15:31:11 GMT
Connection: keep-alive
ETag: "5b91483f-c"
Accept-Ranges: bytes
hello world
-- UPDATE 2 --
Awesome you figured it out, this post seems to explain why:
docker data volume vs mounted host directory
The host directory is, by its nature, host-dependent. For this reason, you can’t mount a host directory from Dockerfile because built images should be portable. A host directory wouldn’t be available on all potential hosts.
If you have some persistent data that you want to share between containers, or want to use from non-persistent containers, it’s best to create a named Data Volume Container, and then to mount the data from it.

Answer 2

I ran across this issue as well when trying to create a volume to place my own nginx configurations into etc/nginx. While I never found out the real issue I think it has to do with how nginx is built from the Dockerfile.
I solved the problem by using my own Dockerfile to extend the original and copy over the configuration files at build time. Hopefully this helps.
FROM nginx:1.15.2
COPY ./nginx.conf /etc/nginx/nginx.conf
COPY ./global /etc/nginx/global
COPY ./conf.d /etc/nginx/conf.d