Update Dockerfile to use ECR, ruby 2.3.1 and vendor/cache

[Omicron7]

No description provided.

[coveralls]

Coverage remained the same at 99.569% when pulling a2aa392 on staging into 346ab97 on master.

[coveralls]

Coverage remained the same at 99.569% when pulling 36addea on staging into 346ab97 on master.

[soberstadt]

We usually don't check in the source files inside of vendor/cache, could you try running bundle package?

[Omicron7]

That was from bundle package --all, adds source gems. Figured this would guarantee we had same version when built on other platforms. Things like github:master could differ between builds.

[soberstadt]

Oh I see now. I would still say that we don't need these in here. With the gemlock file, it should lock down to a specific commit.

[coveralls]

Coverage remained the same at 99.569% when pulling 7ca9603 on staging into 346ab97 on master.