Update docker file to use rocky8

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 --platform=linux/amd64 centos:7
+FROM --platform=linux/amd64 rockylinux:8
 
 WORKDIR /work
 
@@ -8,7 +8,7 @@ COPY buildbucket_environment.yaml .
 COPY package.sh .
 
 # Install the `which` command for debugging
-RUN yum -y install which git && yum clean all
+RUN dnf -y install which git findutils && dnf clean all
 
 # Download and install miniforge
 # https://github.com/conda-forge/miniforge#miniforge