be explicit about our platform in the buildbucket Dockerfile

buildbucket/Dockerfile

 # To build:
 # docker build -t gitlab.ssec.wisc.edu:5555/cspp_geo/cspp-geo-gridded-glm/buildbucket:r$(date '+%Y%m%d') .
-FROM centos:7
+FROM --platform=linux/amd64 centos:7
 
 WORKDIR /work