hostinfo: set Hostinfo.PackageType for mkctr container builds

Fixes tailscale/corp#21448

Change-Id: Id60fb5cd7d31ef94cdbb176141e034845a480a00
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

build_docker.sh

@@ -49,7 +49,7 @@ case "$TARGET" in
         -X tailscale.com/version.gitCommitStamp=${VERSION_GIT_HASH}" \
       --base="${BASE}" \
       --tags="${TAGS}" \
-      --gotags="ts_kube" \
+      --gotags="ts_kube,ts_package_container" \
       --repos="${REPOS}" \
       --push="${PUSH}" \
       --target="${PLATFORM}" \