Configuration fails on read-only root filesystem

In docker-entrypoint there is some sed commands that are creating a temporary file in the folder that file exists. 

But in environments that enforce RO permission, the only available alternative is to mount the entire WEB-INF and create logic to copy files or mount a custom docker-entrypoint. 

The proposal is to create this temporary file inside the XWiki data directory, and change .first_start_completed file creation to not fail if this can't be created. 

For cases where a custom CONTEXT_PATH is used, I don't think there is a solution for RO environments. I will keep only the suggestion for using another alternative.