Update docker instructions

A few things have changed since these Dockerfiles where introduced:
* The choria registry has changed;
* The choria image is now based on AlmaLinux 9;
* Puppet 8 is required to run choria.

While here, make builds somewhat more reproducible by setting an
explicit choria version.

misc/Dockerfile-choria-enroll

@@ -1,11 +1,11 @@
-FROM choria/choria
+FROM registry.choria.io/choria/choria:0.29.4
 
 USER root
 
 # Puppet is required for task input validation
 RUN yum -y install wget && \
- wget 'https://yum.puppetlabs.com/puppet7-release-el-8.noarch.rpm' && \
- rpm -i puppet7-release-el-8.noarch.rpm && \
+ wget 'https://yum.puppetlabs.com/puppet8-release-el-9.noarch.rpm' && \
+ rpm -i puppet8-release-el-9.noarch.rpm && \
  yum -y install puppet-agent
 
 RUN /opt/puppetlabs/puppet/bin/gem install --bindir /opt/puppetlabs/bin choria-mcorpc-support

misc/Dockerfile-mco

@@ -1,11 +1,11 @@
-FROM choria/choria
+FROM registry.choria.io/choria/choria:0.29.4
 
 USER root
 
 # Puppet is required for task input validation
 RUN yum -y install wget && \
- wget 'https://yum.puppetlabs.com/puppet7-release-el-8.noarch.rpm' && \
- rpm -i puppet7-release-el-8.noarch.rpm && \
+ wget 'https://yum.puppetlabs.com/puppet8-release-el-9.noarch.rpm' && \
+ rpm -i puppet8-release-el-9.noarch.rpm && \
  yum -y install puppet-agent
 
 RUN /opt/puppetlabs/puppet/bin/gem install --bindir /opt/puppetlabs/bin choria-mcorpc-support