Fix DNS queries in images with systemd-resolved on hosts without it

On some Toolbx images with systemd-resolved(8), like the fedora-toolbox
images for Fedora 39 onwards, /etc/resolv.conf can end up being a
symbolic link inside the container that expects the host operating
system to also use systemd-resolved(8):
  $ ls -l /etc/resolv.conf
  lrwxrwxrwx. 1 root root 39 Nov 28 08:50 /etc/resolv.conf ->
    ../run/systemd/resolve/stub-resolv.conf

This happens because systemd-resolved(8) already makes /etc/resolv.conf
a symbolic link inside the image, and, hence, the container's entry
point doesn't change it to point at the host's copy of the file at
/run/host/etc/resolv.conf.  Instead, it's left pointing to the host's
copy of the files maintained by systemd-resolved(8) under
/run/systemd/resolve, which happen to be also available inside the
container [1].

If the host OS doesn't use systemd-resolved(8), like Red Hat Enterprise
Linux 9, then this leads to a dangling symbolic link and breaks DNS
queries.

Note that the presence of systemd-resolved(8) in the recent
fedora-toolbox images is a regression caused by the ToolbxReleaseBlocker
Change [2] for Fedora 39 where the image was rewritten in terms of
fedora-kickstarts and pungi-fedora instead of a Container/Dockerfile.
By mistake, systemd crept in as an RPM needed by the image [3], which
in turn pulled in the systemd-resolved RPM as a weak dependency [4].

Hopefully, that will get fixed.  However, it's also not practical to
keep track of all the Toolbx images out there in the wild, so it's
wise to make toolbox(1) more resilient to such things.

This will have the downside of overwriting some custom user-made
modifications to the container's /etc/resolv.conf.  While that's
unfortunate, it's more important to have Toolbx images produce working
containers on a wide range of host OSes.  It will be better to come up
with a more explicit way to support custom user-made modifications to
the container's configuration.  Perhaps with a persistent stamp file.

[1] Commit af602c7
    containers@af602c7d227617d2
    containers#707

[2] https://fedoraproject.org/wiki/Changes/ToolbxReleaseBlocker

[3] fedora-kickstarts commit 48e2c3b5598de32f
    https://pagure.io/fedora-kickstarts/c/48e2c3b5598de32f

[4] fedora-kickstarts commit 49306cb6eada8777
    https://pagure.io/fedora-kickstarts/c/49306cb6eada8777

containers#1410

src/cmd/initContainer.go

@@ -200,7 +200,8 @@ func initContainer(cmd *cobra.Command, args []string) error {
 			return err
 		}
 
-		if _, err := os.Readlink("/etc/resolv.conf"); err != nil {
+		if resolvConfTarget, err := os.Readlink("/etc/resolv.conf"); err != nil ||
+			resolvConfTarget != "/run/host/etc/resolv.conf" {
 			if err := redirectPath("/etc/resolv.conf",
 				"/run/host/etc/resolv.conf",
 				false); err != nil {

test/system/203-network.bats

@@ -62,6 +62,132 @@ teardown() {
   assert [ ${#stderr_lines[@]} -eq 0 ]
 }
 
+@test "network: /etc/resolv.conf inside the default container" {
+  create_default_container
+
+  run --keep-empty-lines --separate-stderr "$TOOLBOX" run readlink /etc/resolv.conf
+
+  assert_success
+  assert_line --index 0 "/run/host/etc/resolv.conf"
+
+  if check_bats_version 1.10.0; then
+    assert [ ${#lines[@]} -eq 1 ]
+  else
+    assert [ ${#lines[@]} -eq 2 ]
+  fi
+
+  # shellcheck disable=SC2154
+  assert [ ${#stderr_lines[@]} -eq 0 ]
+}
+
+@test "network: /etc/resolv.conf inside Arch Linux" {
+  create_distro_container arch latest arch-toolbox-latest
+
+  run --keep-empty-lines --separate-stderr "$TOOLBOX" run --distro arch readlink /etc/resolv.conf
+
+  assert_success
+  assert_line --index 0 "/run/host/etc/resolv.conf"
+
+  if check_bats_version 1.10.0; then
+    assert [ ${#lines[@]} -eq 1 ]
+  else
+    assert [ ${#lines[@]} -eq 2 ]
+  fi
+
+  # shellcheck disable=SC2154
+  assert [ ${#stderr_lines[@]} -eq 0 ]
+}
+
+@test "network: /etc/resolv.conf inside Fedora 34" {
+  create_distro_container fedora 34 fedora-toolbox-34
+
+  run --keep-empty-lines --separate-stderr "$TOOLBOX" run --distro fedora --release 34 readlink /etc/resolv.conf
+
+  assert_success
+  assert_line --index 0 "/run/host/etc/resolv.conf"
+
+  if check_bats_version 1.10.0; then
+    assert [ ${#lines[@]} -eq 1 ]
+  else
+    assert [ ${#lines[@]} -eq 2 ]
+  fi
+
+  # shellcheck disable=SC2154
+  assert [ ${#stderr_lines[@]} -eq 0 ]
+}
+
+@test "network: /etc/resolv.conf inside RHEL 8.7" {
+  create_distro_container rhel 8.7 rhel-toolbox-8.7
+
+  run --keep-empty-lines --separate-stderr "$TOOLBOX" run --distro rhel --release 8.7 readlink /etc/resolv.conf
+
+  assert_success
+  assert_line --index 0 "/run/host/etc/resolv.conf"
+
+  if check_bats_version 1.10.0; then
+    assert [ ${#lines[@]} -eq 1 ]
+  else
+    assert [ ${#lines[@]} -eq 2 ]
+  fi
+
+  # shellcheck disable=SC2154
+  assert [ ${#stderr_lines[@]} -eq 0 ]
+}
+
+@test "network: /etc/resolv.conf inside Ubuntu 16.04" {
+  create_distro_container ubuntu 16.04 ubuntu-toolbox-16.04
+
+  run --keep-empty-lines --separate-stderr "$TOOLBOX" run --distro ubuntu --release 16.04 readlink /etc/resolv.conf
+
+  assert_success
+  assert_line --index 0 "/run/host/etc/resolv.conf"
+
+  if check_bats_version 1.10.0; then
+    assert [ ${#lines[@]} -eq 1 ]
+  else
+    assert [ ${#lines[@]} -eq 2 ]
+  fi
+
+  # shellcheck disable=SC2154
+  assert [ ${#stderr_lines[@]} -eq 0 ]
+}
+
+@test "network: /etc/resolv.conf inside Ubuntu 18.04" {
+  create_distro_container ubuntu 18.04 ubuntu-toolbox-18.04
+
+  run --keep-empty-lines --separate-stderr "$TOOLBOX" run --distro ubuntu --release 18.04 readlink /etc/resolv.conf
+
+  assert_success
+  assert_line --index 0 "/run/host/etc/resolv.conf"
+
+  if check_bats_version 1.10.0; then
+    assert [ ${#lines[@]} -eq 1 ]
+  else
+    assert [ ${#lines[@]} -eq 2 ]
+  fi
+
+  # shellcheck disable=SC2154
+  assert [ ${#stderr_lines[@]} -eq 0 ]
+}
+
+@test "network: /etc/resolv.conf inside Ubuntu 20.04" {
+  create_distro_container ubuntu 20.04 ubuntu-toolbox-20.04
+
+  run --keep-empty-lines --separate-stderr "$TOOLBOX" run --distro ubuntu --release 20.04 readlink /etc/resolv.conf
+
+  assert_success
+  assert_line --index 0 "/run/host/etc/resolv.conf"
+
+  if check_bats_version 1.10.0; then
+    assert [ ${#lines[@]} -eq 1 ]
+  else
+    assert [ ${#lines[@]} -eq 2 ]
+  fi
+
+  # shellcheck disable=SC2154
+  assert [ ${#stderr_lines[@]} -eq 0 ]
+}
+
 @test "network: DNS inside the default container" {
   local ipv4_skip=false
   local ipv4_addr