hg-5.4: forcing pull for hgext3rd images
We can't currently rebuild everything (no-cache) because of #2, Without `no-cache`, the `hg clone` layer gets mounted from another image, that does not have the 5.4 tag. We could have decided to introduce more build options, but found simpler to add an intermediate pull to obtain 5.4 images right away. This can be removed safely once #2 is solved.
topic/default/hg-5.4 in 22 seconds1 job for