Document Ubuntu backports prerequisite (#10375)

* Document Ubuntu backports prerequisite

Add software-properties-common to the Ubuntu backports setup command
and explain why it is needed in minimal/container environments.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Reduce formatting noise in Ubuntu backports docs

Restore the original spacing/layout in the touched markdown files so the PR shows only the Ubuntu backports prerequisite changes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Lee Coward <leecow@microsoft.com>

Author: 3 people

.github/skills/update-distro-packages/SKILL.md

@@ -141,7 +141,7 @@ Optional fields on each release (populated by package availability queries):
   ],
   "dotnet_packages_other": {
     "backports": {
-      "install_command": "sudo add-apt-repository ppa:dotnet/backports && sudo apt-get update",
+      "install_command": "sudo apt-get install -y software-properties-common && sudo add-apt-repository ppa:dotnet/backports && sudo apt-get update",
       "packages": [
         { "component": "sdk", "name": "dotnet-sdk-11.0" }
       ]
@@ -324,7 +324,7 @@ Then `ubuntu.json` release 24.04 becomes:
   ],
   "dotnet_packages_other": {
     "backports": {
-      "install_command": "sudo add-apt-repository ppa:dotnet/backports && sudo apt-get update",
+      "install_command": "sudo apt-get install -y software-properties-common && sudo add-apt-repository ppa:dotnet/backports && sudo apt-get update",
       "packages": [
         { "component": "sdk", "name": "dotnet-sdk-10.0" }
       ]
@@ -425,10 +425,10 @@ When packages come from a non-builtin feed, the `install_command` field tells us
 Feed name: `backports`
 
 ```bash
-sudo add-apt-repository ppa:dotnet/backports && sudo apt-get update
+sudo apt-get install -y software-properties-common && sudo add-apt-repository ppa:dotnet/backports && sudo apt-get update
 ```
 
-This PPA provides .NET packages for older Ubuntu LTS releases that don't carry .NET in the default archive. After registering, packages are installed with the normal `apt-get install` command.
+This PPA provides .NET packages for older Ubuntu LTS releases that don't carry .NET in the default archive. `add-apt-repository` comes from `software-properties-common`, which is often missing in containers and other minimal environments. After registering, packages are installed with the normal `apt-get install` command.
 
 ### Microsoft packages.microsoft.com (PMC)
 

release-notes/10.0/distros/ubuntu.json

@@ -211,7 +211,7 @@
               "name": "dotnet-aspnetcore-runtime-10.0"
             }
           ],
-          "install_command": "sudo add-apt-repository ppa:dotnet/backports && sudo apt-get update"
+          "install_command": "sudo apt-get install -y software-properties-common && sudo add-apt-repository ppa:dotnet/backports && sudo apt-get update"
         }
       }
     }

release-notes/10.0/dotnet-packages.md

@@ -218,7 +218,7 @@ sudo apt-get install -y \
 Register the feed:
 
 ```bash
-sudo add-apt-repository ppa:dotnet/backports && sudo apt-get update
+sudo apt-get install -y software-properties-common && sudo add-apt-repository ppa:dotnet/backports && sudo apt-get update
 ```
 
 ```bash