fix docker.sh

[bi1yeu]

i got errors when running ./docker.sh on M1 macbook.

this change

• uses newer version of Golang image that supports multi-arch
• uses an env flag to disable modules

❯ ./docker.sh
WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested
# _/usr/src/koans
runtime: failed to create new OS thread (have 2 already; errno=22)
fatal error: newosproc

runtime stack:
runtime.throw(0x852790, 0x9)
        /usr/local/go/src/runtime/panic.go:530 +0x90
runtime.newosproc(0xc820028000, 0xc820037fc0)
        /usr/local/go/src/runtime/os1_linux.go:149 +0x18c
runtime.newm(0x8c51f0, 0x0)
        /usr/local/go/src/runtime/proc.go:1513 +0x135
runtime.main.func1()
        /usr/local/go/src/runtime/proc.go:125 +0x2c
runtime.systemstack(0xd74300)
        /usr/local/go/src/runtime/asm_amd64.s:291 +0x79
runtime.mstart()
        /usr/local/go/src/runtime/proc.go:1048

goroutine 1 [running]:
runtime.systemstack_switch()
        /usr/local/go/src/runtime/asm_amd64.s:245 fp=0xc820022770 sp=0xc820022768
runtime.main()
        /usr/local/go/src/runtime/proc.go:126 +0x62 fp=0xc8200227c0 sp=0xc820022770
runtime.goexit()
        /usr/local/go/src/runtime/asm_amd64.s:1998 +0x1 fp=0xc8200227c8 sp=0xc8200227c0
FAIL    _/usr/src/koans [build failed]

❯ ./docker.sh
go: cannot find main module, but found .git/config in /usr/src/koans
        to create a module there, run:
        go mod init