unixPb: Install hostname in sles 15 dockerfile

[Haroon-Khel]

• commit message has one of the standard prefixes
• faq.md updated if appropriate
• other documentation is changed or added (if applicable)
• playbook changes run through VPC or QPC (if you have access)
• VPC/QPC not applicable for this PR
• for inventory.yml changes, bastillion/nagios/jenkins updated accordingly

The hostname command is needed a jdk_net test:
https://ci.adoptium.net/job/Grinder/8802/testReport/sun_net_www_protocol_https_HttpsURLConnection_PostThruProxyWithAuth/java/PostThruProxyWithAuth/

Server started at: [SSL: ServerSocket[addr=60d5d0846858/172.17.0.18,localport=40167]]
Proxy server created: ServerSocket[addr=60d5d0846858/172.17.0.18,localport=38103]
Proxy server listening at: ServerSocket[addr=60d5d0846858/172.17.0.18,localport=38103]
Command line: [hostname]
executeProcess() failed: java.io.IOException: Cannot run program "hostname": error=2, No such file or directory
Client side failed: Get hostname failed.

[steelhead31]

LGTM

[sxa]

I didn't see this before it was merged but this may be a good example of a test to add into the table in the FAQ.md in this respiratory as something that can catch a specific missing letter.