Add Makefile, use Docker for lint/test/CI

[aureliojargas]

Instead of installing dependencies in both the local machine and in
Travis runner, install everything inside the clitest Docker image:

• Supported shells: bash, dash, mksh, zsh
• Linter: checkbashisms

Also updated Alpine version from 3.7 to 3.11.

The image size grew from ~40MB to ~60MB.

The Docker container is not using ENTRYPOINT anymore, to make it easier
to run arbitrary commands inside it. So now you have to explicitly run
the clitest command.

Add a Makefile to isolate all the commands related to dev tasks such as
running linters and tests. Note that by default all those tasks are run
inside the clitest Docker container. To run them directly on the host,
avoiding the container, unset the docker_run variable. Examples:

make test-bash               # test using container's bash
make test-bash docker_run=   # test using host's bash

The CI configuration (Travis) was updated to avoid installing
dependencies and to run make targets inside a freshly built Docker
container.

Also fixed the COLUMNS handling in the tests, so it is compatible with
bash and mksh running inside the container.