Update README.md

[wcalebgray]

After several hours trying to work around the user prompts in my CI environment, I dug in the code and found that if you set these environment variables, you can avoid the prompts. This should be very obvious to users in the documentation and not hidden in the code.

[jspizziri]

@wcalebgray,

I'd be best to include a codeblock example of this in addition to the documentation you added.

[Gl4di4torRr]

This is how it works. I put this in my gulpfile.js

process.env.IMAGE = 'sidekick';
process.env.PUSH = 1;

new GulpDocker(gulp, {
        sidekick: {
                name: "chrisbolton",
                dockerfile: ".",
                tags: ["1"],
                repo: "nexus.mycompany.com:8003/com/mycompany/devops/chrisbolton"
        }
});

[nash10x]

@wcalebgray I am seeing these on the console after introducing the env variables

double free or corruption (out)
SIGABRT: abort
PC=0x7fef69757e97 m=0 sigcode=18446744073709551610
signal arrived during cgo execution

goroutine 1 [syscall, locked to thread]:
runtime.cgocall(0x4afd50, 0xc420047cc0, 0xc420047ce8)
/usr/lib/go-1.8/src/runtime/cgocall.go:131 +0xe2 fp=0xc420047c90 sp=0xc420047c50
github.com/docker/docker-credential-helpers/secretservice._Cfunc_free(0xbcd920)
github.com/docker/docker-credential-helpers/secretservice/_obj/_cgo_gotypes.go:111 +0x41 fp=0xc420047cc0 sp=0xc420047c90
github.com/docker/docker-credential-helpers/secretservice.Secretservice.List.func5(0xbcd920)
/build/golang-github-docker-docker-credential-helpers-cMhSy1/golang-github-docker-docker-credential-helpers-0.5.0/obj-x86_64-linux-gnu/src/github.com/docker/docker-credential-helpers/secretservice/secretservice_linux.go:96 +0x60 fp=0xc420047cf8 sp=0xc420047cc0
github.com/docker/docker-credential-helpers/secretservice.Secretservice.List(0x0, 0x756060, 0xc4200702c0)
/build/golang-github-docker-docker-credential-helpers-cMhSy1/golang-github-docker-docker-credential-helpers-0.5.0/obj-x86_64-linux-gnu/src/github.com/docker/docker-credential-helpers/secretservice/secretservice_linux.go:97 +0x217 fp=0xc420047da0 sp=0xc420047cf8
github.com/docker/docker-credential-helpers/secretservice.(*Secretservice).List(0x77e548, 0xc420047e88, 0x410022, 0xc420070220)
:4 +0x46 fp=0xc420047de0 sp=0xc420047da0
github.com/docker/docker-credential-helpers/credentials.List(0x756ba0, 0x77e548, 0x7560e0, 0xc42008e008, 0x0, 0x10)
/build/golang-github-docker-docker-credential-helpers-cMhSy1/golang-github-docker-docker-credential-helpers-0.5.0/obj-x86_64-linux-gnu/src/github.com/docker/docker-credential-helpers/credentials/credentials.go:145 +0x3e fp=0xc420047e68 sp=0xc420047de0
github.com/docker/docker-credential-helpers/credentials.HandleCommand(0x756ba0, 0x77e548, 0x7ffefef7d7bb, 0x4, 0x7560a0, 0xc42008e000, 0x7560e0, 0xc42008e008, 0x40e398, 0x4d35c0)
/build/golang-github-docker-docker-credential-helpers-cMhSy1/golang-github-docker-docker-credential-helpers-0.5.0/obj-x86_64-linux-gnu/src/github.com/docker/docker-credential-helpers/credentials/credentials.go:60 +0x16d fp=0xc420047ed8 sp=0xc420047e68
github.com/docker/docker-credential-helpers/credentials.Serve(0x756ba0, 0x77e548)
/build/golang-github-docker-docker-credential-helpers-cMhSy1/golang-github-docker-docker-credential-helpers-0.5.0/obj-x86_64-linux-gnu/src/github.com/docker/docker-credential-helpers/credentials/credentials.go:41 +0x1cb fp=0xc420047f58 sp=0xc420047ed8
main.main()
/build/golang-github-docker-docker-credential-helpers-cMhSy1/golang-github-docker-docker-credential-helpers-0.5.0/secretservice/cmd/main_linux.go:9 +0x4f fp=0xc420047f88 sp=0xc420047f58
runtime.main()
/usr/lib/go-1.8/src/runtime/proc.go:185 +0x20a fp=0xc420047fe0 sp=0xc420047f88
runtime.goexit()
/usr/lib/go-1.8/src/runtime/asm_amd64.s:2197 +0x1 fp=0xc420047fe8 sp=0xc420047fe0

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
/usr/lib/go-1.8/src/runtime/asm_amd64.s:2197 +0x1

rax 0x0
rbx 0x7ffefef7cef0
rcx 0x7fef69757e97
rdx 0x0
rdi 0x2
rsi 0x7ffefef7cc80
rbp 0x7ffefef7cff0
rsp 0x7ffefef7cc80
r8 0x0
r9 0x7ffefef7cc80
r10 0x8
r11 0x246
r12 0x7ffefef7cef0
r13 0x1000
r14 0x0
r15 0x30
rip 0x7fef69757e97
rflags 0x246
cs 0x33
fs 0x0
gs 0x0