fix(bin/eqmx): do not rely on existing user name

Container should be able to run as other user too. Use uid as fallback.
Prerequisite to be able to run as user specified during startup.

bin/emqx

@@ -48,7 +48,7 @@ RUNNER_SCRIPT="$RUNNER_BIN_DIR/$REL_NAME"
 CODE_LOADING_MODE="${CODE_LOADING_MODE:-embedded}"
 REL_DIR="$RUNNER_ROOT_DIR/releases/$REL_VSN"
 
-WHOAMI=$(whoami)
+WHOAMI=$(whoami 2>/dev/null || id -u)
 
 # hocon try to read environment variables starting with "EMQX_"
 export HOCON_ENV_OVERRIDE_PREFIX='EMQX_'