Improvements for Docker setup

[Oaphi]

Closes #1398

This PR is an overhaul of our Docker setup to make it more flexible and resilient. It includes:

• change of container restart policy to on-failure:3 (meaning 3 consequent failures cause containers to exit). Our current policy of always sends containers in a restart loop if there is a fatal error, making it hard to debug and resolve issues;
• removed obsolete version field from docker-compose (just getting rid of an annoying warning);
• removed mention of sleep command from Docker docs as there is none (probably legacy?);
• added Dockerfile.dev for setting up Docker specifically for local development purposes;
• when built with Dockerfile.dev, the uwsgi container no longer automatically starts Rails when run;
• made Dockerfile configurable via .env (automatically created by local-setup.sh from compose-env).

Rationale for no longer auto-starting Rails when building with Dockerfile.dev:

• allows for live debugging in case there are issues that cause hard failures (currently, the container exits);
• prevents restart loops in case of hard failures (partially mitigated already by the restart policy change);
• the additional step (connecting to the container & running a single command) is fully documented in README.md.

Rationale for making Dockerfile used for the uwsgi container configurable:

• allows use of custom Docker build configurations as one sees fit as long as they set the CLIENT_DOCKERFILE environment variable;
• required to facilitate the use of Dockerfile.dev for development setups without the need to hard-code the changes;
• is backwards-compatible due to the default value of compose-env set to docker/Dockerfile;