Dockerize it

[veelenga]

Since the shard supports only Linux environment, this PR adds a suggestion how to do the development on other environments (via the Docker). I'm on Mac OSX, so:

hardware git:feat/docker ❯ crystal spec                                                                                                                                                                  
E.

Failures:

  1) Hardware::CPU parses '/proc/stat'

       Error opening file '/proc/stat' with mode 'r': No such file or directory
       /usr/local/Cellar/crystal-lang/0.24.2_1/src/file.cr:33:5 in 'initialize'
       /usr/local/Cellar/crystal-lang/0.24.2_1/src/file.cr:25:3 in 'new'
       /usr/local/Cellar/crystal-lang/0.24.2_1/src/file.cr:427:5 in 'read'
       /usr/local/Cellar/crystal-lang/0.24.2_1/src/file.cr:441:3 in 'read'
       src/hardware/cpu.cr:3:11 in 'info'
       ~procProc(Nil)@spec/cpu_spec.cr:4
       /usr/local/Cellar/crystal-lang/0.24.2_1/src/spec/methods.cr:255:3 in 'it'
       ~procProc(Nil)@spec/cpu_spec.cr:3
       /usr/local/Cellar/crystal-lang/0.24.2_1/src/spec/context.cr:255:3 in 'describe'
       /usr/local/Cellar/crystal-lang/0.24.2_1/src/spec/methods.cr:16:5 in 'describe'
       spec/hardware_spec.cr:1:1 in '__crystal_main'
       /usr/local/Cellar/crystal-lang/0.24.2_1/src/crystal/main.cr:11:3 in '_crystal_main'
       /usr/local/Cellar/crystal-lang/0.24.2_1/src/crystal/main.cr:112:5 in 'main_user_code'
       /usr/local/Cellar/crystal-lang/0.24.2_1/src/crystal/main.cr:101:7 in 'main'
       /usr/local/Cellar/crystal-lang/0.24.2_1/src/crystal/main.cr:135:3 in 'main'

Finished in 175 microseconds
2 examples, 0 failures, 1 errors, 0 pending

Failed examples:

crystal spec spec/cpu_spec.cr:4 

hardware git:master ❯ docker-compose up                                                                                                                                                                 
Starting hardware_spec_1 ... done
Attaching to hardware_spec_1
spec_1  | ........
spec_1  |
spec_1  | Finished in 679 microseconds
spec_1  | 8 examples, 0 failures, 0 errors, 0 pending
hardware_spec_1 exited with code 0

hardware git:master ❯ docker-compose run spec                                                                                                                                                          
........

Finished in 578 microseconds
8 examples, 0 failures, 0 errors, 0 pending

Since the source files are mounted into the container, it is possible to edit the files as usual and run specs in docker.

[bararchy]

@veelenga How about we instead add support for Mac as well?
we can use compile time flags and wrap specific OS logic inside

[veelenga]

We can. I was also thinking about it. But unless we don't support it, the development on OS X is problematic :/

[bararchy]

I see, we can start by having the docker (which always makes sense), and then we start working on adding MacOS support, I'll merge this and open an issue