[CI] Docker push fails on Travis-CI

[asfimport]

See for https://app.travis-ci.com/github/apache/arrow/jobs/551022779#L3226

Pushing ubuntu-cpp (apache/arrow-dev:arm64v8-ubuntu-20.04-cpp)...
The push refers to repository [docker.io/apache/arrow-dev]
ERROR: denied: requested access to the resource is denied

Reporter: Antoine Pitrou / @pitrou

_{Note: This issue was originally created as ARROW-15023. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
cc @kszucs

[asfimport]

Krisztian Szucs / @kszucs:
My guess is that INFRA must update the docker credentials.

[asfimport]

Kouhei Sutou / @kou:
We disabled it because secret variables that are registered at Travis CI were leaked.

We can re-register them at https://app.travis-ci.com/github/apache/arrow/settings .
We don't need to ask INFRA to do it.

[asfimport]

Antoine Pitrou / @pitrou:
How can we do that without exposing those variables? Can they be enabled only on upstream pushes, not PRs, for example?

[asfimport]

Kouhei Sutou / @kou:
I think that the vulnerability has been fixed. So we can set CROSSBOW_GITHUB_TOKEN again safely.

[pitrou]

We have no Travis-CI builds anymore, closing as outdated.