"go get" is broken in alpine image

[avoskoboynik]

This makes alpine image useless for many tasks, including compiling/building.

Example:

alex ~> docker run -it --rm golang:1.11-alpine go get github.com/aws/aws-sdk-go
go: missing Git command. See https://golang.org/s/gogetcmd
package github.com/aws/aws-sdk-go: exec: "git": executable file not found in $PATH
alex ~> 

Same command works fine with standard image:

alex ~> docker run -it --rm golang:1.11 go get github.com/aws/aws-sdk-go
alex ~>                                                                                                                                                                                         

[tianon]

Duplicate of #119, #140, #153, #157, #166, #198, #209, #230, #239.