Add stack config command

Make use of existing modules and functions in order to output the merged configs.
Added skip interpolation flag of variables, so that you can pipe the output back to stack deploy without much hassle.

Signed-off-by: Stoica-Marcu Floris-Andrei <floris.sm@gmail.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/index.md

@@ -152,13 +152,14 @@ read the [`dockerd`](dockerd.md) reference page.
 
 ### Swarm stack commands
 
-| Command                             | Description                                    |
-|:------------------------------------|:-----------------------------------------------|
-| [stack deploy](stack_deploy.md)     | Deploy a new stack or update an existing stack |
-| [stack ls](stack_ls.md)             | List stacks in the swarm                       |
-| [stack ps](stack_ps.md)             | List the tasks in the stack                    |
-| [stack rm](stack_rm.md)             | Remove the stack from the swarm                |
-| [stack services](stack_services.md) | List the services in the stack                 |
+| Command                             | Description                                             |
+|:------------------------------------|:--------------------------------------------------------|
+| [stack deploy](stack_deploy.md)     | Deploy a new stack or update an existing stack          |
+| [stack ls](stack_ls.md)             | List stacks in the swarm                                |
+| [stack ps](stack_ps.md)             | List the tasks in the stack                             |
+| [stack rm](stack_rm.md)             | Remove the stack from the swarm                         |
+| [stack services](stack_services.md) | List the services in the stack                          |
+| [stack config](stack_config.md)     | Output the Compose file after merges and interpolations |
 
 ### Plugin commands
 

docs/reference/commandline/stack_config.md

@@ -0,0 +1,68 @@
+---
+title: "stack config"
+description: "The stack config command description and usage"
+keywords: "stack, config"
+---
+
+# stack config
+
+```markdown
+Usage:	docker stack config [OPTIONS]
+
+Outputs the final config file, after doing merges and interpolations
+
+Aliases:
+  config, cfg
+
+Options:
+  -c, --compose-file strings   Path to a Compose file, or "-" to read from stdin
+      --orchestrator string    Orchestrator to use (swarm|kubernetes|all)
+      --skip-interpolation     Skip interpolation and output only merged config
+```
+
+## Description
+
+Outputs the final Compose file, after doing the merges and interpolations of the input Compose files.
+
+## Examples
+
+The following command outputs the result of the merge and interpolation of two Compose files.
+
+```bash
+$ docker stack config --compose-file docker-compose.yml --compose-file docker-compose.prod.yml
+```
+
+The Compose file can also be provided as standard input with `--compose-file -`:
+
+```bash
+$ cat docker-compose.yml | docker stack config --compose-file -
+```
+
+### Skipping interpolation
+
+In some cases, it might be useful to skip interpolation of environment variables.
+For example, when you want to pipe the output of this command back to `stack deploy`.
+
+If you have a regex for a redirect route in an environment variable for your webserver you would use two `$` signs to prevent `stack deploy` from interpolating `${1}`.
+
+```bash
+  service: webserver
+  environment:
+    REDIRECT_REGEX=http://host/redirect/$${1} 
+```
+
+With interpolation, the `stack config` command will replace the environment variable in the Compose file 
+with `REDIRECT_REGEX=http://host/redirect/${1}`, but then when piping it back to the `stack deploy` 
+command it will be interpolated again and result in undefined behavior. 
+That is why, when piping the output back to `stack deploy` one should always prefer the `--skip-interpolation` option.
+
+```
+$ docker stack config --compose-file web.yml --compose-file web.prod.yml --skip-interpolation | docker stack deploy --compose-file -
+```
+
+## Related commands
+
+* [stack deploy](stack_deploy.md)
+* [stack ps](stack_ps.md)
+* [stack rm](stack_rm.md)
+* [stack services](stack_services.md)

docs/reference/commandline/stack_deploy.md

@@ -111,3 +111,4 @@ axqh55ipl40h  vossibility_vossibility-collector  replicated  1/1       icecrime/
 * [stack ps](stack_ps.md)
 * [stack rm](stack_rm.md)
 * [stack services](stack_services.md)
+* [stack config](stack_config.md)