Issue 777 manage image updates

[maaikelimper]

#777

[maaikelimper]

@tomkralidis as discussed today the update now uses the github-releases as a reference, see function definition here:
https://github.com/wmo-im/wis2box/blob/issue-777-manage-image-updates/wis2box-ctl.py#L128-L163

Note that in order for this approach to work we need to update our release-process and replace the steps using git tag -a x.y.z -m 'tagging release version x.y.z' with creating a github-release, for the images managed outside of the main wis2box-repository