Improve variable interpolation, required flag and default value.

[aramalipoor]

As first proposed in #1377 I think it would be very useful to have a way to set a variable as required, and also set some default value.

web:
   image: org/app:${VERSION?} # Required

Or

web:
   image: org/app:${VERSION:latest} # Default Value

[majidgolshadi]

👍

[dustinblackman]

👍

[arm4b]

👍 for defaults.

[valeriangalliat]

👍

[mseashor]

👍

[srstsavage]

+1

[bennycornelissen]

👍

[plouc]

I'm working on a project where people have heterogeneous env, I'm working on OSX and some are using Ubuntu, when using docker-machine with virtualbox provider, I have to put the full path to make Volume works:

myservice:
  volumes:
    - ${HOST_VOLUME_PATH}/src:/src

The problem is now people who don't use virtualbox have to define the value, it should be great if we could have a default value:

myservice:
  volumes:
    - ${HOST_VOLUME_PATH:.}/src:/src

In other words 👍

[obazoud]

👍

[jlrigau]

👍

[scasagrande-d2l]

+1

[leogamas]

👍