Update dependency composer/installers to v1.12.0

This MR contains the following updates:

Package	Type	Update	Change
composer/installers (source)	require	minor	1.11.0 -> 1.12.0

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Release Notes

composer/installers

v1.12.0

Compare Source

Added

• Added installer for Pantheon Quicksilver modules/scripts #​495.

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Dockerfile

@@ -4,7 +4,7 @@ ADD . /build
 WORKDIR /build
 RUN /build/install.sh
 
-FROM registry.git.autistici.org/ai3/docker/apache2-php-base:s6
+FROM registry.git.autistici.org/ai3/docker/apache2-php-base:master
 
 COPY --from=build /build/app/ /opt/noblogs/www