Implements frontend side of moby#2122.

This adds the syntax to Dockerfile frontend.
I purposedly chose to use a simple format for this as it's likely going
to be debated. As implemented, the following format is supported:

```
RUN --mount=type=secret,id=MYSECRET,env
```
or, more explicitly:

```
RUN --mount=type=secret,id=MYSECRET,env=true
```

will mount the secret with id MYSECRET as a new environment variable with the same name.

Using 'target', it's possible to create a different environment
variable:
```
RUN --mount=type=secret,id=mysecret,target=MY_SECRET,env
```
will mount 'mysecret' secret as MY_SECRET environment variable.

Any suggestions on making it more ergonomic are welcome.

Signed-off-by: a-palchikov <deemok@gmail.com>

frontend/dockerfile/dockerfile2llb/convert_secrets.go

@@ -39,6 +39,12 @@ func dispatchSecret(d *dispatchState, m *instructions.Mount, loc []parser.Range)
 	if !m.Required {
 		opts = append(opts, llb.SecretOptional)
 	}
+	if m.SecretAsEnv {
+		if m.Target == "" {
+			target = path.Base(id)
+		}
+		opts = append(opts, llb.SecretAsEnv(true))
+	}
 
 	if m.UID != nil || m.GID != nil || m.Mode != nil {
 		var uid, gid, mode int

frontend/dockerfile/dockerfile_secrets_test.go

@@ -16,6 +16,8 @@ import (
 var secretsTests = integration.TestFuncs(
 	testSecretFileParams,
 	testSecretRequiredWithoutValue,
+	testSecretAsEnviron,
+	testSecretAsEnvironWithOverride,
 )
 
 func init() {
@@ -80,3 +82,63 @@ RUN --mount=type=secret,required,id=mysecret foo
 	require.Error(t, err)
 	require.Contains(t, err.Error(), "secret mysecret: not found")
 }
+
+func testSecretAsEnviron(t *testing.T, sb integration.Sandbox) {
+	integration.SkipOnPlatform(t, "windows")
+	f := getFrontend(t, sb)
+
+	dockerfile := []byte(`
+FROM busybox
+RUN --mount=type=secret,id=mysecret,env=true [ "$mysecret" == "pw" ] || false
+`)
+
+	dir := integration.Tmpdir(
+		t,
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+	)
+
+	c, err := client.New(sb.Context(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	_, err = f.Solve(sb.Context(), c, client.SolveOpt{
+		LocalMounts: map[string]fsutil.FS{
+			dockerui.DefaultLocalNameDockerfile: dir,
+			dockerui.DefaultLocalNameContext:    dir,
+		},
+		Session: []session.Attachable{secretsprovider.FromMap(map[string][]byte{
+			"mysecret": []byte("pw"),
+		})},
+	}, nil)
+	require.NoError(t, err)
+}
+
+func testSecretAsEnvironWithOverride(t *testing.T, sb integration.Sandbox) {
+	integration.SkipOnPlatform(t, "windows")
+	f := getFrontend(t, sb)
+
+	dockerfile := []byte(`
+FROM busybox
+RUN --mount=type=secret,id=mysecret,target=MY_SECRET,env [ "$MY_SECRET" == "pw" ] || false
+`)
+
+	dir := integration.Tmpdir(
+		t,
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+	)
+
+	c, err := client.New(sb.Context(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	_, err = f.Solve(sb.Context(), c, client.SolveOpt{
+		LocalMounts: map[string]fsutil.FS{
+			dockerui.DefaultLocalNameDockerfile: dir,
+			dockerui.DefaultLocalNameContext:    dir,
+		},
+		Session: []session.Attachable{secretsprovider.FromMap(map[string][]byte{
+			"mysecret": []byte("pw"),
+		})},
+	}, nil)
+	require.NoError(t, err)
+}

frontend/dockerfile/instructions/commands_runmount.go

@@ -122,6 +122,7 @@ type Mount struct {
 	CacheID      string
 	CacheSharing ShareMode
 	Required     bool
+	SecretAsEnv  bool
 	Mode         *uint64
 	UID          *uint64
 	GID          *uint64
@@ -154,6 +155,9 @@ func parseMount(val string, expander SingleWordExpander) (*Mount, error) {
 				m.ReadOnly = false
 				roAuto = false
 				continue
+			case "env":
+				m.SecretAsEnv = true
+				continue
 			case "required":
 				if m.Type == MountTypeSecret || m.Type == MountTypeSSH {
 					m.Required = true
@@ -252,9 +256,15 @@ func parseMount(val string, expander SingleWordExpander) (*Mount, error) {
 				return nil, errors.Errorf("invalid value %s for gid", value)
 			}
 			m.GID = &gid
+		case "env":
+			env, err := strconv.ParseBool(value)
+			if err != nil {
+				return nil, errors.Errorf("invalid value for %q: %q", key, value)
+			}
+			m.SecretAsEnv = env
 		default:
 			allKeys := []string{
-				"type", "from", "source", "target", "readonly", "id", "sharing", "required", "mode", "uid", "gid", "src", "dst", "ro", "rw", "readwrite",
+				"type", "from", "source", "target", "readonly", "id", "sharing", "required", "mode", "uid", "gid", "src", "dst", "ro", "rw", "readwrite", "env",
 			}
 			return nil, suggest.WrapError(errors.Errorf("unexpected key '%s' in '%s'", key, field), key, allKeys, true)
 		}