Bump Gitian build instructions to v0.18.1.0

[sethforprivacy]

[hyc]

Patch looks ok. I wonder if we should instead make the instructions to set a shell variable for the version instead, so we only need to update the doc in one place and just use the variable everywhere else?

VERSION=v0.18.0.0
./dockrun.sh $VERSION

etc...

[jeffro256]

@hyc I think that's the better option and that's how it's done in the main gitian README

[sethforprivacy]

Patch looks ok. I wonder if we should instead make the instructions to set a shell variable for the version instead, so we only need to update the doc in one place and just use the variable everywhere else?

VERSION=v0.18.0.0
./dockrun.sh $VERSION

etc...

Will make and push that change now, good suggestion.