Refacor amazon-ecr-credential-helper usage

[MatthiasScholzTW]

The configuration is cluttered around and it is unclear what is necessary and used.

The docker config.json is at two places:

• ~/.docker/config.json
• /etc/docker/config.json

As well for Amazon Linux 2 LTS now a yum package is available:

• Installation + "-y"

This will facilitate binary installation.

[MatthiasScholzTW]

Reference in the nomad client/server configuration is /etc/docker/config.json