update Dockerfile to use multi-stage build #53
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Before
![mailroom-before](https://user-images.githubusercontent.com/75167930/139134358-4aef5b9a-e69e-4185-a585-252f8df9f507.png)
After
![mailroom-after](https://user-images.githubusercontent.com/75167930/139136010-b4afd560-8212-4612-baa5-37e363a7314c.png)