Allow to disable warning for root user package management

This PR implements the flag that allows to disable the root
warning when using `pip` to install packages. While there are
differing opinions on this, it seems that the final decision is
to lean forward to implement a long and not very easily
discoverable flag to accommodate the minority of users who know
what they are doing and using root installation to - for example
build optimized Dockerfiles.

Even approval, and eventually an adoption of PEP 668 to make the
problem largely go away is still a long time to go, so as
discussed in pypa#10556 (comment)
this PR adds a long and non-user friendly flag to handle the
situation.

Fixes: pypa#10556

news/10990.feature.rst

@@ -0,0 +1 @@
+The install and uinstall commands allow to opt-out from root warning

src/pip/_internal/commands/install.py

@@ -222,6 +222,14 @@ def add_options(self) -> None:
             default=True,
             help="Do not warn about broken dependencies",
         )
+        self.cmd_opts.add_option(
+            "--no-warn-when-using-as-a-root-user-to-manage-os-packages",
+            dest="no_warn_about_root_user",
+            action="store_true",
+            help="Disable warning when there are legitimate uses "
+            "for managing os packages as root user"
+            "(for example in some cases of container builds)",
+        )
 
         self.cmd_opts.add_option(cmdoptions.no_binary())
         self.cmd_opts.add_option(cmdoptions.only_binary())
@@ -464,8 +472,10 @@ def run(self, options: Values, args: List[str]) -> int:
             self._handle_target_dir(
                 options.target_dir, target_temp_dir, options.upgrade
             )
-
-        warn_if_run_as_root()
+        if options.no_warn_about_root_user:
+            pass
+        else:
+            warn_if_run_as_root()
         return SUCCESS
 
     def _handle_target_dir(

src/pip/_internal/commands/uninstall.py

@@ -53,7 +53,14 @@ def add_options(self) -> None:
             action="store_true",
             help="Don't ask for confirmation of uninstall deletions.",
         )
-
+        self.cmd_opts.add_option(
+            "--no-warn-when-using-as-a-root-user-to-manage-os-packages",
+            dest="no_warn_about_root_user",
+            action="store_true",
+            help="Disable warning when there are legitimate uses "
+            "for managing os packages as root user"
+            "(for example in some cases of container builds)",
+        )
         self.parser.insert_option_group(0, self.cmd_opts)
 
     def run(self, options: Values, args: List[str]) -> int:
@@ -100,6 +107,8 @@ def run(self, options: Values, args: List[str]) -> int:
             )
             if uninstall_pathset:
                 uninstall_pathset.commit()
-
-        warn_if_run_as_root()
+        if options.no_warn_about_root_user:
+            pass
+        else:
+            warn_if_run_as_root()
         return SUCCESS