Fix Dockerfile problems

[dpordomingo]

It addresses two problems when building and running docker image:

• building: dep is not a valid command as reported by Documentation improvements src-d/code-annotation#29 (comment)
• running: CMD command does not exist as reported by Documentation improvements src-d/code-annotation#29 (comment)

[smacker]

all this code will be removed in src-d#36

[dpordomingo]

But it's a dependency of src-d#29
Otherwise, the image creation is broken in master, what's a bad thing that needs to be avoided.
Would you prefer if it's pull requested against srcd/master instead of against your working branch?

[smacker]

Section about docker will be removed completely. According to company policy users should use only images created by us and download them from docker hub. Because we don’t have such image yet documentation will be removed.