Document apptainer push

devel-tools/containers/README.md

@@ -10,3 +10,13 @@ apptainer build container.sif container.def
 ```
 apptainer run container.sif
 ```
+
+4. (Optional) Push the container to the GitHub repository's "Packages"
+   section; currently we only have one package, called `devel-containers`,
+   which comes in different versions depending on the OS and software
+   contained.  For example, to update the most commonly used container
+   version:
+```
+apptainer push CentOS9-devel.sif oras://ghcr.io/colvars/devel-containers:CentOS9-devel
+```
+(note that the above requires having set up an access token for apptainer)