Add docker executor

[yohamta]

Implement Docker Executor. See #298.

Example:

steps:
  - name: deno_hello_world
    executor: 
      type: docker
      config:
        image: "denoland/deno:1.10.3"
        autoRemove: true
    command: run https://examples.deno.land/hello-world.ts

[codecov-commenter]

Codecov Report

Base: 82.93% // Head: 82.53% // Decreases project coverage by -0.40% ⚠️

Coverage data is based on head (9f2b043) compared to base (3470e33).
Patch coverage: 37.50% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #363      +/-   ##
==========================================
- Coverage   82.93%   82.53%   -0.41%     
==========================================
  Files          46       46              
  Lines        3580     3612      +32     
==========================================
+ Hits         2969     2981      +12     
- Misses        453      471      +18     
- Partials      158      160       +2     

Impacted Files	Coverage Δ
internal/dag/builder.go	71.94% <37.50%> (-2.87%)	⬇️

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