A few small tweaks and improvements

[NikolausDemmel]

While implementing #21 I did the following minor tweaks and small improvements that are independent of #21, so I provide a separate PR.

• Never use proxy when communicating with the vault.
• remove temporary tared archive of .ssh folder
• make sure /vault/.ssh exists (e.g. in case we start the container with an empty volume)
• make sure ONVAULT works with empty .ssh folder
• Add .dockerignore for all except the actually needed files.

[maxcnunes]

Thanks @NikolausDemmel