Use "grep -xF ... >/dev/null" instead of "grep -qxF ..."

Use "grep -xF ... >/dev/null" instead of "grep -qxF ..."

1. -q "Exit immediately with zero status if any match is found, even if
an error was detected." --grep(1)

Depending on buffering and timing, if grep exits early (before
consuming stdin) "docker images" can receive SIGPIPE and exit
non-zero. We use "set -o pipefail" here, so this fails the "docker
load" phase and then the whole job.

2. "Portable shell scripts should avoid both -q and -s and should
redirect standard and error output to /dev/null instead." --grep(1)

No issue #