docker compose variable substituion

Both $VARIABLE and ${VARIABLE} syntax are supported. Additionally when using the 2.1 file format, it is possible to provide inline default values using typical shell syntax:

• ${VARIABLE:-default} evaluates to default if VARIABLE is unset or empty in the environment.
• ${VARIABLE-default} evaluates to default only if VARIABLE is unset in the environment.

Similarly, the following syntax allows you to specify mandatory variables:

• ${VARIABLE:?err} exits with an error message containing err if VARIABLE is unset or empty in the environment.
• ${VARIABLE?err} exits with an error message containing err if VARIABLE is unset in the environment.