Thanks Zachary for your response. Below are the error messages i got:
ERROR: http://dl-cdn.alpinelinux.org/alpine/v3.12/main: temporary error (try again later)
WARNING: Ignoring APKINDEX.2c4ac24e.tar.gz: No such file or directory
ERROR: http://dl-cdn.alpinelinux.org/alpine/v3.12/community: temporary error (try again later)
WARNING: Ignoring APKINDEX.40a3604f.tar.gz: No such file or directory
2 errors; 35 distinct packages available
ERROR: Service ‘app’ failed to build: The command ‘/bin/sh -c apk update && apk add --update --no-cache postgresql-client’ returned a non-zero code: 2