Fix missing env variable

Author: rnijveld

Dockerfile

@@ -4,7 +4,7 @@ FROM ghcr.io/tweedegolf/debian:bookworm
 ENV POSTGRESQL_VERSION 16
 RUN install -d /usr/share/postgresql-common/pgdg \
     && curl -o /usr/share/postgresql-common/pgdg/apt.postgresql.org.asc --fail https://www.postgresql.org/media/keys/ACCC4CF8.asc \
-    && echo "deb [signed-by=/usr/share/postgresql-common/pgdg/apt.postgresql.org.asc] http://apt.postgresql.org/pub/repos/apt/ $DEBIAN_VERSION-pgdg main" > /etc/apt/sources.list.d/pgdg.list \
+    && echo "deb [signed-by=/usr/share/postgresql-common/pgdg/apt.postgresql.org.asc] http://apt.postgresql.org/pub/repos/apt/ bookworm-pgdg main" > /etc/apt/sources.list.d/pgdg.list \
     && apt-get update \
     && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
         postgresql-client-$POSTGRESQL_VERSION \