Make Dockerfile more resilient.

Also changes the password that is used for MediaWiki setup from
'admin' to 'linktitles'.

install-composer.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+# This is a slightly modified version of the installer script published at
+# https://getcomposer.org/doc/faqs/how-to-install-composer-programmatically.md
+# It does not depend on `wget` and uses `curl` instead.
+
+EXPECTED_CHECKSUM="$(curl -q https://composer.github.io/installer.sig)"
+php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
+ACTUAL_CHECKSUM="$(php -r "echo hash_file('sha384', 'composer-setup.php');")"
+
+if [ "$EXPECTED_CHECKSUM" != "$ACTUAL_CHECKSUM" ]
+then
+    >&2 echo 'ERROR: Invalid installer checksum'
+    rm composer-setup.php
+    exit 1
+fi
+
+# May need to remove the explicit pinning of version 1 in the future
+php composer-setup.php --quiet --1
+RESULT=$?
+rm composer-setup.php
+exit $RESULT