There are a lot of ways to confuse Compose with a badly-formed docker-compose.yml. This file should be checked when it is read and good error messages should be produced to help users fix the problem.

This could be done by producing a schema for the file, like was done in #1348.

These sorts of things should be checked:

• Top-level object is a dict
• Each key (a service name) maps to a dict (Improve error when service is not a dict #128)
• String config options (e.g. image) - are numbers allowed? When? Don't accept YAML's eager interpretation of values (e.g. true) which should be quoted (docker-compose does not pass environment variables true/false unchanged #1788)
• List config options (e.g. volumes) either must be a list, or we helpfully wrap single values in a list for you
• String-or-list config options (e.g. command, entrypoint)
• Dictionary config options (e.g. environment)
• Format of various options (e.g. volumes - see Error when specifying permission in volume #260)

References: #117 #127