fix: dockerfile FromAs casing (#4616)

Dockerfile

@@ -60,7 +60,7 @@ RUN --mount=type=cache,sharing=private,target=/root/.cargo/registry \
 # https:/actions/runner-images/blob/main/images/linux/Ubuntu2004-Readme.md#cached-docker-images
 ##
 # A slim image contains only forest binaries
-FROM ubuntu:22.04 as slim-image
+FROM ubuntu:22.04 AS slim-image
 
 ARG SERVICE_USER=forest
 ARG SERVICE_GROUP=forest
@@ -93,7 +93,7 @@ RUN forest -V && forest-cli -V && forest-tool -V
 ENTRYPOINT ["forest"]
 
 # A fat image contains forest binaries and fil proof parameter files under $FIL_PROOFS_PARAMETER_CACHE
-FROM slim-image as fat-image
+FROM slim-image AS fat-image
 
 # Move FIL_PROOFS_PARAMETER_CACHE out of forest data dir since users always need to mount the data dir
 ENV FIL_PROOFS_PARAMETER_CACHE="/var/tmp/filecoin-proof-parameters"