Actually fix docker compose in updater

updater/Dockerfile

@@ -1,6 +1,19 @@
 # Use the official PHP image with Apache
 FROM php:8.1-apache
 
+# Add Docker's official GPG key:
+RUN apt-get update &&\
+    apt-get install ca-certificates curl &&\
+    install -m 0755 -d /etc/apt/keyrings &&\
+    curl -fsSL https://download.docker.com/linux/debian/gpg -o /etc/apt/keyrings/docker.asc &&\
+    chmod a+r /etc/apt/keyrings/docker.asc
+
+# Add the repository to Apt sources:
+RUN echo \
+      "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/debian \
+      $(. /etc/os-release && echo "$VERSION_CODENAME") stable" | \
+      tee /etc/apt/sources.list.d/docker.list > /dev/null
+
 # Install git and docker-compose
 RUN apt-get update && \
     apt-get install -y git docker-compose-plugin && \