Docker command provided in readme fails

Running

```
docker run --rm -v `pwd`:/go/src/in -v `pwd`:/out gopy/gopy app bind -output=/out in
```

from the root of the repository results in this error:

```
panic: could not look-up "interface{}"!

goroutine 1 [running]:
github.com/go-python/gopy/bind.(*symtab).addSliceType(0xc420160360, 0xc420011630, 0x838ac0, 0xc420011810, 0x834540, 0xc420014ff0, 0x8, 0xc4201604a0, 0x1f, 0xc4200156a0, ...)
/go/src/github.com/go-python/gopy/bind/symtab.go:552 +0x9fc
github.com/go-python/gopy/bind.(*symtab).addType(0xc420160360, 0x838ac0, 0xc420011810, 0x834540, 0xc420014ff0)
/go/src/github.com/go-python/gopy/bind/symtab.go:389 +0x1213
github.com/go-python/gopy/bind.(*symtab).processTuple(0xc420160360, 0xc42000b140)
/go/src/github.com/go-python/gopy/bind/symtab.go:357 +0xe3
github.com/go-python/gopy/bind.(*symtab).addSymbol(0xc420160360, 0x8387a0, 0xc420011860)
/go/src/github.com/go-python/gopy/bind/symtab.go:337 +0xb73
github.com/go-python/gopy/bind.(*Package).process(0xc42009a960, 0x0, 0xc42006bad0)
/go/src/github.com/go-python/gopy/bind/package.go:182 +0x1f2c
github.com/go-python/gopy/bind.NewPackage(0xc420011630, 0xc4200b9960, 0x0, 0x0, 0x0)
/go/src/github.com/go-python/gopy/bind/package.go:47 +0x2c6
main.newPackageFrom(0xc420088a80, 0xc420011630, 0x2, 0xc420011630, 0x0)
/go/src/app/gen.go:222 +0xeb
main.newPackage(0x7ffd3612ef4c, 0x2, 0xc42001491c, 0x4, 0x0)
/go/src/app/gen.go:199 +0x5b5
main.gopyRunCmdBind(0xc4200ae6c0, 0xc42000e130, 0x1, 0x1, 0x0, 0x0)
/go/src/app/cmd_bind.go:82 +0x3b9
github.com/gonuts/commander.(*Command).Dispatch(0xc4200ae6c0, 0xc42000e120, 0x2, 0x2, 0x1, 0x7f544dab5000)
/go/src/github.com/gonuts/commander/commands.go:221 +0x20a
github.com/gonuts/commander.(*Command).Dispatch(0xc4200ae7e0, 0xc42000e110, 0x3, 0x3, 0x0, 0x0)
/go/src/github.com/gonuts/commander/commands.go:187 +0x6a4
main.run(0xc42000e110, 0x3, 0x3, 0xc42003bf78, 0x671709)
/go/src/app/main.go:35 +0x297
main.main()
/go/src/app/main.go:43 +0x6c
```

Also, I'm. not sure if it's related but `bind` is not currently part of the CLI but it's used in the command sent to Docker. Looking at historical issues `bind` seems to be part of an older backwards-incompatible version of the CLI

Start by checking the README command against the current CLI entry points, especially cmd_bind.go and gen.go, and compare it with the reported stack trace through bind/symtab.go and package.go. Reproduce the exact Docker command, then update the documented invocation or the CLI behavior so the command runs successfully from the repository root.

Written by the indexing model from the issue text.