Change the quiet flag behavior in the build command

Right now, the quiet (-q, --quiet) flag ignores the output
generated from within the container.

However, it ought to be quiet in a way that all kind
of diagnostic output should be ignored, unless the build
process fails.

This patch makes the quiet flag behave in the following way:
 1. If the build process succeeds, stdout contains the image ID
    and stderr is empty.
 2. If the build process fails, stdout is empty and stderr
    has the error message and the diagnostic output of that process.

If the quiet flag is not set, then everything goes to stdout
and error messages, if there are any, go to stderr.

Signed-off-by: Boaz Shuster <ripcurld.github@gmail.com>

contrib/completion/fish/docker.fish

@@ -95,7 +95,7 @@ complete -c docker -A -f -n '__fish_seen_subcommand_from build' -l force-rm -d '
 complete -c docker -A -f -n '__fish_seen_subcommand_from build' -l help -d 'Print usage'
 complete -c docker -A -f -n '__fish_seen_subcommand_from build' -l no-cache -d 'Do not use cache when building the image'
 complete -c docker -A -f -n '__fish_seen_subcommand_from build' -l pull -d 'Always attempt to pull a newer version of the image'
-complete -c docker -A -f -n '__fish_seen_subcommand_from build' -s q -l quiet -d 'Suppress the verbose output generated by the containers'
+complete -c docker -A -f -n '__fish_seen_subcommand_from build' -s q -l quiet -d 'Suppress the build output and print image ID on success'
 complete -c docker -A -f -n '__fish_seen_subcommand_from build' -l rm -d 'Remove intermediate containers after a successful build'
 complete -c docker -A -f -n '__fish_seen_subcommand_from build' -s t -l tag -d 'Repository name (and optionally a tag) to be applied to the resulting image in case of success'
 

docs/reference/commandline/build.md

@@ -30,7 +30,7 @@ parent = "smn_cli"
       --memory-swap=""                Total memory (memory + swap), `-1` to disable swap
       --no-cache=false                Do not use cache when building the image
       --pull=false                    Always attempt to pull a newer version of the image
-      -q, --quiet=false               Suppress the verbose output generated by the containers
+      -q, --quiet=false               Suppress the build output and print image ID on success
       --rm=true                       Remove intermediate containers after a successful build
       --shm-size=[]                   Size of `/dev/shm`. The format is `<number><unit>`. `number` must be greater than `0`.  Unit is optional and can be `b` (bytes), `k` (kilobytes), `m` (megabytes), or `g` (gigabytes). If you omit the unit, the system uses bytes. If you omit the size entirely, the system uses `64m`.
       -t, --tag=[]                    Name and optionally a tag in the 'name:tag' format

man/docker-build.1.md

@@ -81,7 +81,7 @@ set as the **URL**, the repository is cloned locally and then sent as the contex
    Always attempt to pull a newer version of the image. The default is *false*.
 
 **-q**, **--quiet**=*true*|*false*
-   Suppress the verbose output generated by the containers. The default is *false*.
+   Suppress the build output and print image ID on success. The default is *false*.
 
 **--rm**=*true*|*false*
    Remove intermediate containers after a successful build. The default is *true*.