Fix go build so the binary runs in alpine container

[max-rocket-internet]

Currently the docker image does not function at all:

$ docker run drone/drone-admit-members
standard_init_linux.go:211: exec user process caused "no such file or directory"

This is because the binary inside is not compiled for alpine:

$ docker run --entrypoint="/bin/sh" -it drone/drone-admit-members
/ # ls /bin/drone-admit-members
/bin/drone-admit-members
/ # /bin/drone-admit-members
/bin/sh: /bin/drone-admit-members: not found
/ # ldd /bin/drone-admit-members
	/lib64/ld-linux-x86-64.so.2 (0x7f373dffe000)
	libpthread.so.0 => /lib64/ld-linux-x86-64.so.2 (0x7f373dffe000)
	libc.so.6 => /lib64/ld-linux-x86-64.so.2 (0x7f373dffe000)

So here I am:

• Adding required env vars to build a binary to run in the alpine docker image
• Fixing build step to use golang:1.12 since this version is in go.mod

[bradrydzewski]

good catch, thanks!