General Repo checking

[Cobrijani]

• Added repository existence that works also for other providers.
• Added additional info if repository is being created.
• Moved tail to CMD part so it is possible to call other restic methods.
• Added container-structure-test for dockerfile

[mrclschstr]

I think this should fix #28 as well.

[mrclschstr]

Btw: The method of checking the repository existence is recommended in the official docs. See: https://restic.readthedocs.io/en/latest/075_scripting.html#check-if-a-repository-is-already-initialized

[niondir]

Wow thanks for the great work. Looks good for me.