Variable substitution not working on Windows 10 with docker compose - docker

Question

I'm wondering if I've stumbled on a bug or that there's something not properly documented about variable substitution on Windows in combination with Docker Machine and Compose (installed version of docker is 1.11.1).
If I run the "docker-compose up" command for a yml file that looks like this:
volumes:
- ${FOOBAR}/build/:/usr/share/nginx/html/
And this variable doesn't exist docker compose will correctly complain about it:
The foobar variable is not set. Defaulting to a blank string.
However, when I change it to an existing environment variable:
volumes:
- ${PROJECT_DIR}/build/:/usr/share/nginx/html/
It will then not properly start the container and displays the following error (trying to access the nginx container will give you a host is unreachable message):
ERROR: for nginx rpc error: code = 2 desc = "oci runtime error: could not synchronise with container process: not a directory"
If I run the echo command in the Docker Quickstart Terminal it will output the correct path that I've set in the environment variable. If I replace the ${PROJECT_DIR} with the environment variable value the container runs correctly.
I get the same type of error message if I try to use the environment variable for the official php image instead of the official nginx image. In both cases the docker compose file works if I substitute ${PROJECT_DIR} text with the content of the environment variable.
So is this a bug or am I missing something?
After some mucking about I've managed to get the containers to start correctly without error messages if I use the following (contains the full path to the local files):
volumes:
- ${PROJECT_DIR}:/usr/share/nginx/html/
The nginx container is then up and running though it cannot find the files then anymore. If I replace the variable with the path it contains it then can find the files again.
Above behaviour isn't consistent. When I added a second environment variable for substitution it gave the oci runtime error. Kept giving it when I removed that second variable and only started working again when I also removed the first variable. After that it suddenly accepted ${PROJECT_DIR}/build/ but still without finding files.
Starting a bash session to the nginx container shows that the mount point for the volume contains no files.
I'm really at a loss here what docker is doing and what it expects from me. Especially as I have no idea to what it is expanding the variables in the compose file.

Answer 1

In the end the conclusion is that variable substitution is too quirky on Windows with Docker Machine to be useful. However, there is an alternative to variable substitution.
If you need a docker environment that does the following:
Can deploy on different computers that don't run the same OS
Doesn't care if the host uses Docker natively or via Virtual Box (this can require path changes)
Then your best bet is to use extending.
First you create the docker-compose.yml file that contains the images you'll need. For example an php image with MySQL:
php:
image: 5.5-apache
links:
- php_db:mysql
- maildev:maildev
ports:
- 8080:80
php_db:
image: mariadb
ports:
- 3306:3306
environment:
MYSQL_ROOT_PASSWORD: examplepass
You might notice that there aren't any volumes defined in this docker-compose file. That is something we're going to define in a file called docker-compose.override.yml:
php:
volumes:
- /workspaces/Eclipse/project/:/var/www/html/
When you have both files in one directory docker-compose does something interesting. It combines them into one adding/overwriting settings in the docker-compose.yml with those present in docker-compose.override.yml.
Then when running the command docker-compose up it will result in a docker run that is configured for the machine you're working on.
You can get similar behaviour with custom files names if you change a few things in your docker-compose command:
docker-compose -f docker-compose.yml -f docker-compose.conf.yml up
The detail is that docker-compose can accept multiple compose files and it will combine them into one. This happens from left to right.
Both methods allows you to create a basic compose file that configures the containers you need. You then can override/add the settings you need for the specific computer you're running docker on.
The page Overview of docker-compose CLI has more details on how these commands work.