docker build has hardcoded timestamp

[edublancas]

#116 discussed an approach, so fix the problem with the {{timestamp}} placeholder and proposed having two placeholders, one that is evaluated when running soopervisor export and another one at task build time.

For the first case, we evaluate the timestamp and store it in an env.yaml file in the container. However, this causes an issue since the timestamp will be hard coded and requires an image rebuild to update the timestamp.

We should think of a way to keep the timestamp at export time but allow it to be recomputed without re-building the image.

FYI @feribg