Move env variables outsude docker build #PR2

[kzaitsev]

#749

[shawnzhu]

@Bugagazavr do you think it's a good idea to move environment variables into container configuration? so that all commands in build scripts (including git clone) can benefit from this. see https://github.com/drone/drone/blob/master/shared/build/build.go#L317

[kzaitsev]

@shawnzhu Conversely, the configuration does not occur at the level of the container. At script.

[shawnzhu]

@Bugagazavr those injected environment variables during container creation will be available within container just like those exported ones in script. my point is removing export command which can be done via script stanza in .drone.yml in anyway.

[kzaitsev]

@shawnzhu in the future, we will delete building container scenario ( https://github.com/drone/drone/blob/master/shared/build/build.go#L427-L465 ), and then we will create container from image ( any image ). Maybe this will be chnged, it is an intermediate option