[PicaPlume] Use ARG to define the image version

pica-plume/Dockerfile

+ARG PLUME_VERSION=v0.4.0
 FROM plumeorg/plume:${PLUME_VERSION}
 
-ENV PLUME_VERSION=v0.4.0
+ENV PLUME_VERSION=${PLUME_VERSION}
 ENV MIGRATION_DIRECTORY=migrations/postgres
 ENV USE_HTTPS=1
 ENV ROCKET_ADDRESS=0.0.0.0