Allow arbitrary --user values

[tianon]

Closes #53 (carried)

This is implemented in the same way we've supported arbitrary values for docker run --user in many other images.

See also docker-library/rabbitmq#60, docker-library/cassandra#48, docker-library/mongo#81, redis/docker-library-redis#48, docker-library/mysql#161, and MariaDB/mariadb-docker#59, docker-library/percona#21.

[tianon]

$ docker run -it --rm --user "$RANDOM:$RANDOM" --name ghost c0704a28c3d5
npm info it worked if it ends with ok
npm info using npm@2.15.11
npm info using node@v4.7.0
npm info prestart ghost@0.11.3
npm info start ghost@0.11.3

> ghost@0.11.3 start /usr/src/ghost
> node index

WARNING: Ghost is attempting to use a direct method to send email. 
It is recommended that you explicitly configure an email service.
Help and documentation can be found at http://support.ghost.org/mail.

Migrations: Creating tables...
Migrations: Creating table: posts
Migrations: Creating table: users
Migrations: Creating table: roles
Migrations: Creating table: roles_users
Migrations: Creating table: permissions
Migrations: Creating table: permissions_users
Migrations: Creating table: permissions_roles
Migrations: Creating table: permissions_apps
Migrations: Creating table: settings
Migrations: Creating table: tags
Migrations: Creating table: posts_tags
Migrations: Creating table: apps
Migrations: Creating table: app_settings
Migrations: Creating table: app_fields
Migrations: Creating table: clients
Migrations: Creating table: client_trusted_domains
Migrations: Creating table: accesstokens
Migrations: Creating table: refreshtokens
Migrations: Creating table: subscribers
Migrations: Running fixture populations
Migrations: Creating owner
Ghost is running in development... 
Listening on 0.0.0.0:2368 
Url configured as: http://localhost:2368 
Ctrl+C to shut down

$ docker top ghost
UID                 PID                 PPID                C                   STIME               TTY                 TIME                CMD
27628               25665               25650               3                   15:10               pts/8               00:00:00            npm
27628               25902               25665               0                   15:10               pts/8               00:00:00            sh -c node index
27628               25903               25902               23                  15:10               pts/8               00:00:04            node index

[tianon]

Ok, this is weird:

$ docker run -it --rm --name ghost c0704a28c3d5
ln: failed to create symbolic link '/usr/src/ghost/config.js': Permission denied

(running it as root, thus getting the gosu behavior rather than simply running as a different user)

Not sure yet why that'd cause any difference at all.

[ghost]

May I suggest an update of the documentation to reflect this possibility?
And how do we set the '--user' when using docker-compose?

[yosifkit]

docker-compose would just be user: 1000 or user: 999:50, so it uses the same single string as --user.

[tianon]

Ok, this is updated now. 👍

(Creating the symlink at build-time instead of run-time.)