dockerfile: add support for RUN --mount=type=ssh

* Needs to be compiled with `dfrunmount dfssh`
* Implemented options:
 * `type`(required): needs to be `ssh`
 * `target`(optional): the socket path in the container
 * `id`(optional): id

Test script:

	#!/bin/bash
	set -exu -o pipefail
	REF=localhost:5000/dfssh:latest
	ssh-add -l
	sudo buildctl build --frontend=dockerfile.v0 --local context=. --local dockerfile=frontend/dockerfile/cmd/dockerfile-frontend \
	  --frontend-opt "build-arg:BUILDTAGS=dfrunmount dfssh" \
	  --exporter=image --exporter-opt name=$REF --exporter-opt push=true
	mkdir -p /tmp/foo
	cd /tmp/foo
	cat << EOF > Dockerfile
	# syntax=$REF
	FROM alpine
	RUN apk add --no-cache openssh-client
	RUN mkdir -p -m 0700 ~/.ssh && ssh-keyscan gitlab.com >> ~/.ssh/known_hosts
	RUN --mount=type=ssh ssh git@gitlab.com
	# "Welcome to GitLab, @GITLAB_USERNAME_ASSOCIATED_WITH_SSHKEY" should be printed here
	EOF
	sudo buildctl build --ssh default=$SSH_AUTH_SOCK --progress=plain --frontend=dockerfile.v0 --local context=. --local dockerfile=.

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

client/llb/exec.go

@@ -496,6 +496,12 @@ func SSHID(id string) SSHOption {
 	})
 }
 
+func SSHSocketTarget(target string) SSHOption {
+	return sshOptionFunc(func(si *SSHInfo) {
+		si.Target = target
+	})
+}
+
 func SSHSocketOpt(target string, uid, gid, mode int) SSHOption {
 	return sshOptionFunc(func(si *SSHInfo) {
 		si.Target = target

frontend/dockerfile/dockerfile2llb/convert_nossh.go

@@ -0,0 +1,13 @@
+// +build dfrunmount,!dfssh
+
+package dockerfile2llb
+
+import (
+	"github.com/moby/buildkit/client/llb"
+	"github.com/moby/buildkit/frontend/dockerfile/instructions"
+	"github.com/pkg/errors"
+)
+
+func dispatchSSH(m *instructions.Mount) (llb.RunOption, error) {
+	return nil, errors.Errorf("ssh mounts not allowed")
+}

frontend/dockerfile/dockerfile2llb/convert_runmount.go

@@ -65,6 +65,14 @@ func dispatchRunMounts(d *dispatchState, c *instructions.RunCommand, sources []*
 			out = append(out, secret)
 			continue
 		}
+		if mount.Type == instructions.MountTypeSSH {
+			ssh, err := dispatchSSH(mount)
+			if err != nil {
+				return nil, err
+			}
+			out = append(out, ssh)
+			continue
+		}
 		if mount.ReadOnly {
 			mountOpts = append(mountOpts, llb.Readonly)
 		}

frontend/dockerfile/dockerfile2llb/convert_ssh.go

@@ -0,0 +1,27 @@
+// +build dfssh dfextall
+
+package dockerfile2llb
+
+import (
+	"github.com/moby/buildkit/client/llb"
+	"github.com/moby/buildkit/frontend/dockerfile/instructions"
+	"github.com/pkg/errors"
+)
+
+func dispatchSSH(m *instructions.Mount) (llb.RunOption, error) {
+	if m.Source != "" {
+		return nil, errors.Errorf("ssh does not support source")
+	}
+	opts := []llb.SSHOption{llb.SSHID(m.CacheID)}
+
+	if m.Target != "" {
+		// TODO(AkihiroSuda): support specifying permission bits
+		opts = append(opts, llb.SSHSocketTarget(m.Target))
+	}
+
+	if !m.Required {
+		opts = append(opts, llb.SSHOptional)
+	}
+
+	return llb.AddSSHSocket(opts...), nil
+}

frontend/dockerfile/instructions/commands_nossh.go

@@ -0,0 +1,7 @@
+// +build !dfssh,!dfextall
+
+package instructions
+
+func isSSHMountsSupported() bool {
+	return false
+}

frontend/dockerfile/instructions/commands_runmount.go

@@ -14,12 +14,14 @@ const MountTypeBind = "bind"
 const MountTypeCache = "cache"
 const MountTypeTmpfs = "tmpfs"
 const MountTypeSecret = "secret"
+const MountTypeSSH = "ssh"
 
 var allowedMountTypes = map[string]struct{}{
 	MountTypeBind:   {},
 	MountTypeCache:  {},
 	MountTypeTmpfs:  {},
 	MountTypeSecret: {},
+	MountTypeSSH:    {},
 }
 
 const MountSharingShared = "shared"
@@ -47,6 +49,11 @@ func isValidMountType(s string) bool {
 			return false
 		}
 	}
+	if s == "ssh" {
+		if !isSSHMountsSupported() {
+			return false
+		}
+	}
 	_, ok := allowedMountTypes[s]
 	return ok
 }

frontend/dockerfile/instructions/commands_ssh.go

@@ -0,0 +1,7 @@
+// +build dfssh dfextall
+
+package instructions
+
+func isSSHMountsSupported() bool {
+	return true
+}

hack/test

@@ -16,4 +16,5 @@ docker run --rm $iid go build ./frontend/dockerfile/cmd/dockerfile-frontend
 
 docker run --rm $iid go build -tags dfrunmount ./frontend/dockerfile/cmd/dockerfile-frontend
 docker run --rm $iid go build -tags "dfrunmount dfsecrets" ./frontend/dockerfile/cmd/dockerfile-frontend
+docker run --rm $iid go build -tags "dfrunmount dfssh" ./frontend/dockerfile/cmd/dockerfile-frontend
 docker run --rm $iid go build -tags dfextall ./frontend/dockerfile/cmd/dockerfile-frontend