Unpin postgresql-client version in Dockerfile

This is so that this (probably) also works
on future os and postgres updates.

Author: k-nut

Dockerfile

@@ -6,6 +6,6 @@ RUN pip install -r requirements.txt
 
 # Install pg_isready to await db start
 RUN apt-get update && \
-    apt-get install postgresql-client-11 --yes --no-install-recommends && \
+    apt-get install postgresql-client --yes --no-install-recommends && \
     rm -rf /var/lib/apt/lists/*