message: Install drush8 in /opt and set drush fallback.
diff: @@ -11,12 +11,14 @@ ENV DRUPAL_CONSOLE_LAUNCHER_VERSION=1.9.1 \
  DRUPAL_CONSOLE_LAUNCHER_SHA=c44be5772de751a498374b43290c693e6a8c79f4 \
  DRUSH_VERSION=8.3.0 \
  DRUSH_LAUNCHER_VERSION=0.6.0 \
  - DRUSH_LAUNCHER_FALLBACK='/home/.composer/vendor/bin/drush'
  + DRUSH_LAUNCHER_FALLBACK=/opt/drush8/vendor/bin/drush
  RUN curl -L -o /usr/local/bin/drupal "https://github.com/hechoendrupal/drupal-console-launcher/releases/download/${DRUPAL_CONSOLE_LAUNCHER_VERSION}/drupal.phar" \
  && echo "${DRUPAL_CONSOLE_LAUNCHER_SHA} /usr/local/bin/drupal" | sha1sum \
  && chmod +x /usr/local/bin/drupal \
  - && php -d memory_limit=-1 /usr/local/bin/composer global require drush/drush:${DRUSH_VERSION} \
  + && mkdir -p /opt/drush8 \
  + && php /usr/local/bin/composer init -n -d /opt/drush8 --require=drush/drush:${DRUSH_VERSION} \
  + && php -d memory_limit=-1 /usr/local/bin/composer update -n -d /opt/drush8 \
  && wget -O /usr/local/bin/drush "https://github.com/drush-ops/drush-launcher/releases/download/${DRUSH_LAUNCHER_VERSION}/drush.phar" \
  && chmod +x /usr/local/bin/drush \
  && mkdir -p /home/.drush
  