Magento's composer plugin hasn't been updated in over a year, they merged a composer 7 fix but haven't released it... So we'll pin the version for now.
@@ -93,7 +93,8 @@ RUN git clone --branch v0.4.15 --depth=1 https://github.com/NoiseByNorthwest/php
&& make install
RUN curl -sS https://getcomposer.org/installer | \
- php -- --install-dir=/usr/local/bin --filename=composer
+ php -- --version=2.6.6 --install-dir=/usr/local/bin --filename=composer
+
COPY conf/blackfire.ini $PHP_INI_DIR/conf.d/blackfire.ini
COPY conf/spx.ini $PHP_INI_DIR/conf.d/spx.ini
Stoyvo