Add mongodb driver to Dockerfile

[plutov]

Currently if I am using Docker and migrate/migrate I am getting:

 error: database driver: unknown driver mongodb (forgotten import?)

[coveralls]

Pull Request Test Coverage Report for Build 309

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 59.188%

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Totals
Change from base Build 298:	0.0%
Covered Lines:	773
Relevant Lines:	1306

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 309

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 59.188%

---

Totals
Change from base Build 298:	0.0%
Covered Lines:	773
Relevant Lines:	1306

---

💛 - Coveralls

[dhui]

Thanks for the PR!