Dockerfile: reduce verbosity (#993)
Dockerfile: reduce verbosity (#993)

* Use the `--quiet=2` option for apt-get commands (note that this
  implies `-y` for install).

* Add the `--quiet` option to composer, and use `update` instead of
  `install` as per the instructions in the warning that we don't use
  a composer.lock file.

* Redirect stdout of `docker-php-ext-install` commands to /dev/null,
  as recommended by the docker-php maintainer (see, e.g.,
  https://github.com/docker-library/php/issues/319).

* Redirect stdout of xdebug installation to /dev/null, because there
  is no recommended way to make the command quieter. While some of
  its stdout is useful (e.g. notification when a new xdebug version
  is available), there is simply too much compiler output.
smrealms/smr,smrealms/smr,smrealms/smr,hemberger/smr,hemberger/smr,hemberger/smr,hemberger/smr