Dockerfile provider (#329)

Co-authored-by: CrazyMax <crazy-max@users.noreply.github.com>

pkg/dockerfile/client.go

@@ -0,0 +1,77 @@
+package dockerfile
+
+import (
+	"bytes"
+	"io/ioutil"
+
+	"github.com/moby/buildkit/frontend/dockerfile/instructions"
+	"github.com/moby/buildkit/frontend/dockerfile/parser"
+	"github.com/moby/buildkit/frontend/dockerfile/shell"
+	"github.com/pkg/errors"
+)
+
+// Client represents an active dockerfile object
+type Client struct {
+	ast      *parser.Node
+	stages   []instructions.Stage
+	metaArgs []instructions.KeyValuePairOptional
+	shlex    *shell.Lex
+}
+
+// Options holds dockerfile client object options
+type Options struct {
+	Filename string
+}
+
+// New initializes a new dockerfile client
+func New(opts Options) (*Client, error) {
+	b, err := ioutil.ReadFile(opts.Filename)
+	if err != nil {
+		return nil, errors.Wrapf(err, "Cannot read Dockerfile %s", opts.Filename)
+	}
+
+	parsed, err := parser.Parse(bytes.NewReader(b))
+	if err != nil {
+		return nil, errors.Wrapf(err, "Cannot parse Dockerfile %s", opts.Filename)
+	}
+
+	stages, metaArgs, err := instructions.Parse(parsed.AST)
+	if err != nil {
+		return nil, errors.Wrapf(err, "Cannot parse stages for Dockerfile %s", opts.Filename)
+	}
+
+	var kvpoArgs []instructions.KeyValuePairOptional
+	shlex := shell.NewLex(parsed.EscapeToken)
+	for _, cmd := range metaArgs {
+		for _, metaArg := range cmd.Args {
+			if metaArg.Value != nil {
+				*metaArg.Value, _ = shlex.ProcessWordWithMap(*metaArg.Value, metaArgsToMap(kvpoArgs))
+			}
+			kvpoArgs = append(kvpoArgs, metaArg)
+		}
+	}
+
+	return &Client{
+		ast:      parsed.AST,
+		stages:   stages,
+		metaArgs: kvpoArgs,
+		shlex:    shlex,
+	}, nil
+}
+
+func (c *Client) isStageName(name string) bool {
+	for _, stage := range c.stages {
+		if stage.Name == name {
+			return true
+		}
+	}
+	return false
+}
+
+func metaArgsToMap(metaArgs []instructions.KeyValuePairOptional) map[string]string {
+	m := map[string]string{}
+	for _, arg := range metaArgs {
+		m[arg.Key] = arg.ValueString()
+	}
+	return m
+}