Use credentials store for docker login from travis

[eine]

WARNING! Your password will be stored unencrypted in /home/travis/.docker/config.json.
Configure a credential helper to remove this warning. See
https://docs.docker.com/engine/reference/commandline/login/#credentials-store

[Nic30]

For what the password is for?

docker has secrets https://docs.docker.com/engine/swarm/secrets/#intermediate-example-use-secrets-with-a-nginx-service

and travis itself can ecrypt keys https://docs.travis-ci.com/user/encryption-keys/

[eine]

It is used during the deploy task of some travis-ci jobs, in order to push docker images to dockerhub. E.g.:

• Pull ghdl/ext:vunit. No credentials required.
• Build ghdl/ext:ide. No credentials required.
• Push ghdl/ext:ide to dockerhub. Credentials required.

Actually, we are using the travis encryption feature you referred. More precisely, this is the same info worded for Docker: https://docs.travis-ci.com/user/docker/#Pushing-a-Docker-Image-to-a-Registry
And this is a equivalent minimum example: https://docs.travis-ci.com/user/build-stages/share-docker-image/

However, the message I pasted in the first comment is shown in every build. See lines 529-531: https://travis-ci.org/1138-4EB/hwd-ide/jobs/366242954