Use pnpm in the monorepo root

[MAILPOET-4485]

dev/php80/Dockerfile

@@ -7,9 +7,10 @@ ARG GID=1000
 RUN apt-get update \
 	&& apt-get install -y git zlib1g-dev libzip-dev zip wget gnupg msmtp libpng-dev gettext subversion \
 	&& \
-    # Install NodeJS + NPM
+    # Install NodeJS, enable Corepack
     curl -sL https://deb.nodesource.com/setup_17.x | bash - && \
     apt-get install -y nodejs build-essential && \
+    corepack enable && \
 	\
 	# Install WP-CLI
 	curl -o /usr/local/bin/wp https://raw.githubusercontent.com/wp-cli/builds/gh-pages/phar/wp-cli.phar && \