Fix database healthcheck (#318)

Because the PostgreSQL user is not always "postgres", do not specify a
user to the Healthcheck command. It will also works correctly.

Closes #313

db/Dockerfile

@@ -19,7 +19,7 @@ RUN apk add --no-cache \
 COPY setup-wale.sh /docker-entrypoint-initdb.d/
 
 #Healthcheck to make sure container is ready
-HEALTHCHECK CMD pg_isready -U postgres || exit 1
+HEALTHCHECK CMD pg_isready || exit 1
 
 # Add and configure entrypoint and command
 COPY entrypoint.sh /