fix: ensure /var/lib/postgresql/data doesn't exist is system containers (#246)

Signed-off-by: Niccolò Fei <niccolo.fei@enterprisedb.com>

Debian/Dockerfile.template

@@ -29,6 +29,7 @@ COPY requirements.txt /
 
 # Install additional extensions
 RUN set -xe; \
+	rm -fr /var/lib/postgresql/data; \
 	apt-get update; \
 	if apt-get -s upgrade | grep "^Inst postgres"; then \
 		echo "ERROR: Upgradable postgres packages found!"; \