CentOS8 branch fails docker build step

[green-nsk]

I'm not very familiar with docker, maybe someone can help me with this.

As an aside, I noticed that centos8 branch isn't up to date with master branch, maybe worth merging (though it doesn't help with this issue).

/usr/bin/docker build -t 4cd98f:b[3](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:3)9b7d76a660[4](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:4)d2e8614[5](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:5)[6](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:6)10a[7](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:7)4cd2b[8](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:8) -f "/home/runner/work/_actions/naveenrajm7/rpmbuild/centos8/Dockerfile" "/home/runner/work/_actions/naveenrajm7/rpmbuild/centos8"
  Sending build context to Docker daemon  70.66kB
  
  Step 1/7 : FROM centos:8
  8: Pulling from library/centos
  a1d0c7532777: Pulling fs layer
  a1d0c7532777: Verifying Checksum
  a1d0c7532777: Download complete
  a1d0c7532777: Pull complete
  Digest: sha256:a27fd8080b517143cbbbab[9](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:9)dfb7c8571c40d67d534bbdee55bd6c473f432b177
  Status: Downloaded newer image for centos:8
   ---> 5d0da3dc9764
  Step 2/7 : COPY . .
   ---> 0[10](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:10)5f8a2cb83
  Step 3/7 : RUN yum install -y rpm-build rpmdevtools gcc make coreutils python
   ---> Running in 97dffab89649
  CentOS Linux 8 - AppStream                      [12](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:12)2  B/s |  38  B     00:00    
  Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in mirrorlist
  The command '/bin/sh -c yum install -y rpm-build rpmdevtools gcc make coreutils python' returned a non-zero code: 1
  
  Warning: Docker build failed with exit code 1, back off 7.227 seconds before retry.
  /usr/bin/docker build -t 4cd98f:b39b7d76a6604d2e86[14](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:14)5610a74cd2b8 -f "/home/runner/work/_actions/naveenrajm7/rpmbuild/centos8/Dockerfile" "/home/runner/work/_actions/naveenrajm7/rpmbuild/centos8"
  Sending build context to Docker daemon  70.66kB
  
  Step 1/7 : FROM centos:8
   ---> 5d0da3dc9764
  Step 2/7 : COPY . .
   ---> Using cache
   ---> 0105f8a2cb83
  Step 3/7 : RUN yum install -y rpm-build rpmdevtools gcc make coreutils python
   ---> Running in 2c5dbeb95d30
  CentOS Linux 8 - AppStream                       57  B/s |  38  B     00:00    
  Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in mirrorlist
  The command '/bin/sh -c yum install -y rpm-build rpmdevtools gcc make coreutils python' returned a non-zero code: 1
  
  Warning: Docker build failed with exit code 1, back off 1.105 seconds before retry.
  /usr/bin/docker build -t 4cd98f:b39b7d76a6604d2e86145610a74cd2b8 -f "/home/runner/work/_actions/naveenrajm7/rpmbuild/centos8/Dockerfile" "/home/runner/work/_actions/naveenrajm7/rpmbuild/centos8"
  Sending build context to Docker daemon  70.66kB
  
  Step 1/7 : FROM centos:8
   ---> 5d0da3dc9764
  Step 2/7 : COPY . .
   ---> Using cache
   ---> 0105f8a2cb83
  Step 3/7 : RUN yum install -y rpm-build rpmdevtools gcc make coreutils python
   ---> Running in 4997af90dbad
  CentOS Linux 8 - AppStream                       [18](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:18)  B/s |  [38](https://github.com/GraystoneTechnologies/exanic-software/runs/7744925425?check_suite_focus=true#step:2:38)  B     00:02    
  Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in mirrorlist
  The command '/bin/sh -c yum install -y rpm-build rpmdevtools gcc make coreutils python' returned a non-zero code: 1
  
Error: Docker build failed with exit code 1