Alternative control commands: bash, Makefile

[nuest]

Currently the ERC is focused on Docker and implementations must construct the execution command themselves, most importantly to support substitution.

Alternatively, the ERC could support bash commands (similar to a Travis config file) or Makefile - based execution. In case for this, the following draft of control commands can be extended:

Implementations SHOULD support a list of [bash](https://en.wikipedia.org/wiki/Bash_(Unix_shell)) commands as control statements.
These commands are given as a list under the node `cmd` under the root-level node `execution`.
Non-bash commands MUST be defined under own nodes under the `execution` node.
The current/working directory for these commands MUST be the [ERC base directory](#base-directory).

The execution commands MAY ensure the re-computation is independent from the environment.
For example, the time zone could be fixed via an environment variable `TZ=CET`, so output formatting of timestamps does not break [checking](../glossary.md#check).
This is in addition to ERC creators handling reproducibility at a script level.

!!! tip "Examples for control statements"
    ```yml
    execution:
      cmd:
        - `./prepare.sh --input my_data`
        - `./execute.sh --output results --iterations 3`
    ```
    
    ```yml
    id: 12345
    execution:
      cmd: >-
        'docker run -it --rm --volume $(pwd):/erc --volume $(pwd)/other_data.csv:/erc/data.csv:ro -e TZ=CET erc:12345'
    ```