podman_guide: Add note about firewall

[james-smith-za]

Add a note reminding the reader that in order for their container service to be useful, firewall settings may need to be adjusted.

I'm a Debian native so I found having a firewall enabled by default non-obvious, and I thought a note might save some head-scratching from other unwashed like myself who are trying to come over to the light side.

Note: I included a link to the firewalld-for-beginners guide but I'm not sure what that's going to look like rendered. Not sure if I went up the right number of ..s.

Author checklist (Completed by original Author)

• Good fit for the Rocky Linux project? Title and Author Metatags inserted ?
• If applicable, steps and instructions have been tested to work
• Initial self-review to fix basic typos and grammar completed

Rocky Documentation checklist (Completed by Rocky team)

• 1st Pass (Document is good fit for project and author checklist completed)
• 2nd Pass (Technical Review - check for technical correctness)
• 3rd Pass (Detailed Editorial Review and Peer Review)
• Final approval (Final Review)

[github-actions]

Test results for c27be73:

Number of broken URLs: 6

URL,RESULT,FILENAME
 https://stigviewer.com/stigs/red_hat_enterprise_linux_8,failed,books/disa_stig/disa_stig_part1.md
 https://stigviewer.com/stigs/apache_server_2.4_unix_server,failed,books/disa_stig/disa_stig_part3.md
 http://your_ip,failed,guides/cms/mediawiki.md
 http://$(hostname):8080,failed,guides/repositories/pulp_fetch_upload.md
 https://ftp.gnu.org/gnu/hello/hello-2.12.tar.gz,failed,labs/systems_administration_I/lab7-software_management.md
 https://repocompare.rockylinux.org,failed,release_notes/8_5.md

[sspencerwire]

@james-smith-za I think your backup is correct, but we can fix it if not. Thanks!

[sspencerwire]

@all-contributors add @james-smith-za for content

[allcontributors]

@sspencerwire

I've put up a pull request to add @james-smith-za! 🎉

[james-smith-za]

Thanks @sspencerwire!