docker.bash: work correctly with multi-ip containers (#5028)

Currently if container under test has multiple IP addresses,
the `docker_ip` function from `docker.sh` will return a gibberish.
This patch makes it return the first address found.
Additionally, I apply shellcheck on `docker.sh`.

fstest/testserver/testserver.go

@@ -91,7 +91,7 @@ func start(name string) error {
 				continue
 			}
 
-			// fs.Debugf(name, "key = %q, envKey = %q, value = %q", key, envKey, value)
+			// fs.Debugf(name, "key = %q, envKey = %q, value = %q", key, envKey(name, string(key)), value)
 			err = os.Setenv(envKey(name, string(key)), string(value))
 			if err != nil {
 				return err