Move composer build to it's own builder

server/Dockerfile

@@ -1,3 +1,8 @@
+FROM composer as composer-build
+    WORKDIR /tmp
+    ADD https://raw.githubusercontent.com/MISP/MISP/v2.4.118/app/composer.json /tmp
+    RUN composer install --ignore-platform-reqs
+
 FROM debian:buster-slim as php-build
     RUN apt-get update; apt-get install -y --no-install-recommends \
         gcc \
@@ -99,11 +104,9 @@ ARG PHP_VER=20180731
 # PHP
     # Install ssdeep prebuild, latest composer, then install the app's PHP deps
     COPY --from=php-build /usr/lib/php/${PHP_VER}/ssdeep.so /usr/lib/php/${PHP_VER}/ssdeep.so
-    COPY --from=composer:latest /usr/bin/composer /usr/bin/composer
-    WORKDIR /var/www/MISP/app
-    RUN composer install \
-    # Enable CakeResque with php-redis
-        ;phpenmod redis \
+    COPY --from=composer-build /tmp/Vendor /var/www/MISP/app/Vendor
+    COPY --from=composer-build /tmp/Plugin /var/www/MISP/app/Plugin
+    RUN phpenmod redis \
     # Enable CakeResque with php-gnupgp
         ;phpenmod gnupg \
     # Enable ssdeep we build earlier