Remake the Docker API

[integraledelebesgue]

Rationale

A so-called "Docker Widgets" (removed in #104) used to consist of sending strings through WebSockets.
Such a solution has some drawbacks:

• Relies on textual serialization which is a total waste of time-space
• Not type-safe
• Does not necessarily support IPC (shared memory...)
• Limited to Docker - it would be nice to support other engines
• Rebuilds images every time
• Not very elegant nor useful

Idea

• Think about improving this concept by leveraging shared memory, binary serialization and type-safety.
• Designing a special Executor similar to the ProcessPoolExecutor but using Docker processes instead of the OS ones.