bug#42996: icecat can escape from `guix environment --container`

[luhux]

I am using guix environment --container to isolate some programs that are prone 
to leak information. guix environment --container works well in freerdp and 
other programs until I use guix environment --container to containerize icecat,

Steps to reproduce:

guix environmnt --container (...some options...) --ad-hoc icecat

Select the address bar and write:'file://' and then access, icecat can still 
access the content outside the container.

Please forgive me for some inappropriate words. My English is not very good.

luhux