update Ubuntu installation instruction

[Slach]

Signed-off-by: Slach [email protected]

close #42
I hope DCO exists

[Slach]

/assign @haircommander

[haircommander]

you may need to rebase off of master again. the DCO failure is on a merge commit, which shouldn't be included in this PR (it's already merged)

[haircommander]

thanks for the PR @Slach BTW

[Slach]

@haircommander could you suggest me
which git commands should I use to rebase with master?
I tried git pull --rebase upstream master
but bot show me label "dco-commit:no" instead of apply rebase

[haircommander]

what I do is

git fetch upstream
git rebase upstream/master
git push -f origin ubuntu_install

[Slach]

@haircommander done, thanks for instruction

[haircommander]

/approve
/lgtm

[openshift-ci-robot]

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