Update alpine image to 3.7

[NicoTexas]

Please update the Postgres alpine images to Alpine 3.7.
Right now they are based on Alpine 3.5

From

FROM alpine:3.5

to

FROM alpine:3.7

[yosifkit]

Duplicate #400.

[NicoTexas]

So, no plan to upgrade underlying Alpine version for 9.5.x, or 9.6.x?