Map the host UID into the container

Assuming a host UID of 1000, the UID mapping inside the user namespace
created by rootless podman for the toolbox container was:
         0       1000          1
         1     100000      65536

... which was the same as seen from the host:
         0       1000          1
         1     100000      65536

Therefore, when running with an UID of 1000 inside the container, it
got mapped to UID 100999 on the host. That means, for example, files
created by the user inside the container end up looking funny from the
host.

This is addressed by creating another user namespace that's a child of
the initial user namespace created by rootless podman. Assuming a host
UID of 1000, the UID mapping inside this child namespace is:
      1000          0          1
         0          1       1000
      1001       1001      64536

... which when seen from the host is:
      1000       1000          1
         0     100000       1000
      1001     101000      64536

This means that UID 1000 inside the child namespace is mapped to the
same UID 1000 on the host via the intermediate namespace created by
rootless podman. UIDs 0 to 999 inside the child namespace are mapped
to UIDs 100000 to 100999 in the host.

This change requires this runc pull request to work:
opencontainers/runc#1862

As suggested by Giuseppe Scrivano.

fedora-toolbox

@@ -88,6 +88,9 @@ create()
         fi
     fi
 
+    max_uid_count=65536
+    max_minus_uid=$((max_uid_count-UID))
+    uid_plus_one=$((UID+1))
     if ! podman create \
                  --hostname toolbox \
                  --interactive \
@@ -96,6 +99,9 @@ create()
                  --privileged \
                  --security-opt label=disable \
                  --tty \
+                 --uidmap $UID:0:1 \
+                 --uidmap 0:1:$UID \
+                 --uidmap $uid_plus_one:$uid_plus_one:$max_minus_uid \
                  --volume $HOME:$HOME \
                  --volume $XDG_RUNTIME_DIR:$XDG_RUNTIME_DIR \
                  $toolbox_image \