So it won't be possible to use a compose-file and stdin at the same time? This would be interesting when people want to overwrite stuff from compose-file without creating a new file. Such as:

$cat my-compose.yml 
web:
  ports: 1234:5000
$ docker-compose up -f my-compose.yml --stdin-extends '{web: {ports: 5000:5000}}'

Which would result in running web with ports 5000:5000 instead of 1234.
(I know the format is wrong but you get the idea ;-) )

@aanm that's correct, the stdin would replace the file. I think this is pretty standard for unix utilities. If you need something from a file, it would be up to you to read the original file, and merge it into the final config you send to stdin.

@aanm What you describe is interesting (and partly covered by the extends option), but orthogonal to the idea of reading input from stdin - it'd apply just as much in the case of having two files, rather than one file + stdin.

@dnephin Come to think of it, what's the behaviour of extends in config read from stdin? Looks like it'll resolve paths relative to the current directory, but that should be documented somewhere.

True, I should add that with the small doc change I made about using - for file.

@aanand I thinking that could some some users problems when they want to scale
something like:

count=0
while true;
let count=count+1
docker-compose up -f my-compose.yml --stdin-extends '{web: {ports: $count:5000}}'
do

My point is too use environment variables has input for configuration in compose. I think that would solve some problems that users are getting.

@dnephin Does docker itself has this feature that you are PR?

@aanm For environment variable interpolation, see #1377.

Updated the docs, and cleaned up get_config_path()

Rebased again! Should be ready

LGTM