Update paquet name changed in strech -> buster

pica-etherpad/Dockerfile

@@ -68,7 +68,7 @@ FROM base
 
 COPY entrypoint.sh /opt/etherpad-lite/entrypoint.sh
 RUN apt-get update && \
-    apt-get install -y mysql-client unoconv libreoffice-writer libreoffice-pdfimport && \
+    apt-get install -y default-mysql-client unoconv libreoffice-writer libreoffice-pdfimport && \
     chmod +x /opt/etherpad-lite/entrypoint.sh && \
     mkdir -p /opt/etherpad-lite && \
     chown -R node /opt/etherpad-lite && \