ARG do not work when it has 'as'

[acekingke]

I write a dockerfile like that:

ARG XTRABACKUP_VER=2.4.28
...
FROM --platform=$TARGETPLATFORM bitnami/percona-xtrabackup:${XTRABACKUP_VER}  as xtrabackupBuilder
...
COPY --from xtrabackupBuilder xxx xxx

It always get error like that:

ERROR: failed to solve: failed to parse stage name "bitnami/percona-xtrabackup:": invalid reference format