Fix PHP Composer installer

Release v0.1.1 is broken, because the PHP Composer installer has changed
and the checksum was hardcoded.

Fix: always grep most recent installer's SHA384 checksum from
https://composer.github.io/installer.sig

Author: rtucek

Dockerfile

@@ -192,7 +192,7 @@ RUN \
 
     # Install PHP composer
     php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');" && \
-    php -r "if (hash_file('SHA384', 'composer-setup.php') === 'aa96f26c2b67226a324c27919f1eb05f21c248b987e6195cad9690d5c1ff713d53020a02ac8c217dbf90a7eacc9d141d') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;" && \
+    php -r "if (hash_file('SHA384', 'composer-setup.php') === trim(file_get_contents('https://composer.github.io/installer.sig'))) { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;" && \
     php composer-setup.php --install-dir=/usr/local/bin --filename=composer && \
     php -r "unlink('composer-setup.php');" && \