docker-compose bind mounts

docker-compose bind mounts will fail if the source does not exist

The volume bind mount type for docker-compose requires that the source directory/file exist before the directory/file is mounted. There are scenarios where you want to mount credentials, for example, .aws. This can keep the credentials out of a checked in docker-compose.yml and leave it to the responsibility of the developer.

Example configuration:

...
    - type: bind
      source: ~/.aws
      target: /root/.aws
      consistency: cached
      volume:
        nocopy: true
...

When using docker in a third-party CI/CD pipeline, this can cause errors like:

...
ERROR: for scratch-backpack-app  Cannot create container for service app: invalid mount config for type "bind": bind source path does not exist: /home/travis/.aws

ERROR: for app  Cannot create container for service app: invalid mount config for type "bind": bind source path does not exist: /home/travis/.aws
...

Since the home directory does not contain .aws the docker-compose command fails.

One solution:

...
    - type: bind
      source: ${AWS_CREDENTIALS_DIR:-~/.aws}
      target: /root/.aws
      consistency: cached
      volume:
        nocopy: true
...

Leveraging shell substitution syntax, this will look for an environment variable AWS_CREDENTIALS_DIR and use it if it exists. If it does not exist, it will default to ~/.aws

Et voila, it is done!