[CE] Rename base image to OVERLEAF_BASE_TAG

SHARELATEX_BASE_TAG was renamed to OVERLEAF_BASE_TAG
in the cloudbuild files in https://github.com/overleaf/internal/pull/16823, but the docker image
was never updated.

GitOrigin-RevId: 8690623addafd5d76ffb310c863abbbf72dbb938

server-ce/Dockerfile

@@ -2,8 +2,8 @@
 # Overleaf Community Edition (overleaf/overleaf)
 # ---------------------------------------------
 
-ARG SHARELATEX_BASE_TAG=sharelatex/sharelatex-base:latest
-FROM $SHARELATEX_BASE_TAG
+ARG OVERLEAF_BASE_TAG=sharelatex/sharelatex-base:latest
+FROM $OVERLEAF_BASE_TAG
 
 WORKDIR /overleaf