Define syntax to fix [1]:

  protoc --go_out=./go config.proto
  [libprotobuf WARNING google/protobuf/compiler/parser.cc:492] No
    syntax specified for the proto file. Please use 'syntax =
    "proto2";' or 'syntax = "proto3";' to specify a syntax
    version. (Defaulted to proto2 syntax.)

Drop 'optional' (with 'sed -i 's/\toptional /\t/' *.proto') to fix:

  protoc --go_out=./go config.proto
  config.proto:9:18: Explicit 'optional' labels are disallowed in the
    Proto3 syntax. To define 'optional' fields in Proto3, simply
    remove the 'optional' label, as fields are 'optional' by default.

Replace the User extensions with 'Any' [2,3] to fix:

  protoc --go_out=./go config.proto
  config.proto: Extensions in proto3 are only allowed for defining
    options.

Drop required (with 'sed -i 's/\trequired /\t/' *.proto') to fix:

  protoc --go_out=./go runtime_config.proto
  runtime_config.proto: Required fields are not allowed in proto3.

Drop DefaultState to fix:

  protoc --go_out=./go runtime_config.proto
  runtime_config.proto: Explicit default values are not allowed in
    proto3.

There's still some trouble with the resulting Go:

  go run ./example.go
  go/config.pb.go:26:8: cannot find package "google/protobuf" in any of:
          /usr/lib/go/src/google/protobuf (from $GOROOT)
          /home/wking/.local/lib/go/src/google/protobuf (from $GOPATH)
  Makefile:31: recipe for target 'example' failed

But I haven't been able to figure that out yet.

[1]: https://developers.google.com/protocol-buffers/docs/proto3#simple
[2]: https://developers.google.com/protocol-buffers/docs/proto3#any
[3]: protocolbuffers/protobuf#828

Signed-off-by: W. Trevor King <wking@tremily.us>

Using jsonpb [1,2].

Drop the proto.String bits to avoid errors like:

  ./example.go:17: cannot use proto.String("linux") (type *string) as
    type string in field value

And use a reference to s to avoid:

  ./example.go:29: cannot use s (type oci.LinuxSpec) as type
     proto.Message in argument to marshaler.Marshal: oci.LinuxSpec
     does not implement proto.Message (ProtoMessage method has pointer
     receiver)

[1]: golang/protobuf#44
[2]: http://godoc.org/github.com/golang/protobuf/jsonpb

Signed-off-by: W. Trevor King <wking@tremily.us>

To avoid:

  go run ./example.go
  go/config.pb.go:26:8: cannot find package "google/protobuf" in any of:
          /usr/lib/go/src/google/protobuf (from $GOROOT)
          /home/wking/.local/lib/go/src/google/protobuf (from $GOPATH)
  Makefile:31: recipe for target 'example' failed

The Dockerfile changes will compile Google's any.proto to Go and
install it in the GOPATH.  Unfortunately, the jsonpb rendering isn't
quite right, since it's spitting out:

  "user": {
    "type_url": "oci.LinuxUser",
    "value": "qAYBsAYBuAYFuAYG"
  },

When it should be spitting out [1]:

  "user": {
    "@type": "type.googleapis.com/oci.LinuxUser",
    "uid": 1,
    "gid": 1,
    "additionalGids": [
      5,
      6
    ]
  },

[1]: https://developers.google.com/protocol-buffers/docs/proto3#json

Signed-off-by: W. Trevor King <wking@tremily.us>

It became the default with golang/protobuf@9ebc6c4e (2015-10-19) and
was removed completely with golang/protobuf@8a5d8e8b (jsonpb: Remove
Marshaler.EnumsAsString, 2015-10-21).

Signed-off-by: W. Trevor King <wking@tremily.us>