For some reason the image didn't build for me correctly as it couldn't find drush. Linking it from a different place helped to fix it (line 38) change:
&& ln -s /.composer/vendor/drush/drush/drush /usr/local/bin/drush \
to
&& ln -s /.config/composer/vendor/drush/drush/drush /usr/local/bin/drush \
For some reason the image didn't build for me correctly as it couldn't find drush. Linking it from a different place helped to fix it (line 38) change:
&& ln -s /.composer/vendor/drush/drush/drush /usr/local/bin/drush \
to&& ln -s /.config/composer/vendor/drush/drush/drush /usr/local/bin/drush \