Make default input files comply to sequence-like block structure

Description

dominic/dune-blocklab!44 (merged) updated the block structure of dune-blocklab to be a sequence instead of a mapping, in compliance with the YAML standard. With this update, all default input YAML files of this repository became outdated and have to be adjusted.

Proposal

Make default input files obey the new sequence block structure. The update is relatively simple:

# OLD:
blocks:
  <type>:
    # ..

# NEW:
blocks
  -
    type: <type>
    # ..

This requires an update of the Docker base image to incorporate the changes to the master of dune-blocklab.