Add --target to bud command

Signed-off-by: TomSweeneyRedHat <[email protected]>

Add the --target option to the bud command.  This allows the
user to specify the last stage to build in a multi stage
Dockerfile.

Addresses containers#632

Tests:
```
FROM fedora:latest
RUN touch /foo

FROM alpine:latest AS mytarget
RUN touch /1

STEP 1: FROM alpine:latest AS mytarget
Getting image source signatures
Copying blob 6c40cc604d8e: 2.63 MiB / 2.63 MiB [============================] 2s
Copying config caf27325b298: 1.48 KiB / 1.48 KiB [==========================] 0s
Writing manifest to image destination
Storing signatures
STEP 2: RUN touch /1
STEP 3: COMMIT containers-storage:[overlay@/var/lib/containers/storage+/var/run/containers/storage]localhost/tom:latest
Getting image source signatures
Skipping blob 503e53e365f3 (already present): 5.52 MiB / 5.52 MiB [=========] 0s
Copying blob 3266af627e70: 3.50 KiB / 3.50 KiB [============================] 0s
Copying config 52c07ff42762: 704 B / 704 B [================================] 0s
Writing manifest to image destination
Storing signatures
--> 52c07ff42762c07286ecb608c342df4af7105190ce30f70b678ff667a26c1842
STEP 4: COMMIT containers-storage:[overlay@/var/lib/containers/storage+/var/run/containers/storage]localhost/tom:latest
Getting image source signatures
Skipping blob 503e53e365f3 (already present): 5.52 MiB / 5.52 MiB [=========] 0s
Skipping blob 3266af627e70 (already present): 3.50 KiB / 3.50 KiB [=========] 0s
Copying config c75fd71d1f59: 704 B / 704 B [================================] 0s
Writing manifest to image destination
Storing signatures
--> c75fd71d1f59c500b2009151919b3ee183c0314db9e91f30e348daca6bbaa74b

IMAGE NAME                                               IMAGE TAG            IMAGE ID             CREATED AT             SIZE
docker.io/library/alpine                                 latest               caf27325b298         Jan 30, 2019 17:19     5.8 MB
localhost/tom

```

Signed-off-by: TomSweeneyRedHat <[email protected]>

cmd/buildah/bud.go

@@ -269,6 +269,7 @@ func budCmd(c *cobra.Command, inputArgs []string, iopts budResults) error {
 		RemoveIntermediateCtrs:  iopts.Rm,
 		ForceRmIntermediateCtrs: iopts.ForceRm,
 		BlobDirectory:           iopts.BlobCache,
+		Target:                  iopts.Target,
 	}
 
 	if iopts.Quiet {

contrib/completions/bash/buildah

@@ -414,6 +414,7 @@ return 1
      --signature-policy
      -t
      --tag
+     --target
      --ulimit
      --userns
      --userns-uid-map

docs/buildah-bud.md

@@ -385,6 +385,12 @@ Specifies the name which will be assigned to the resulting image if the build
 process completes successfully.
 If _imageName_ does not include a registry name, the registry name *localhost* will be prepended to the image name.
 
+**--target** *stageName*
+
+Set the target build stage to build.  When building a Dockerfile with multiple build stages, --target
+can be used to specify an intermediate build stage by name as a final stage for the resulting image.
+Commands after the target stage will be skipped.
+
 **--tls-verify** *bool-value*
 
 Require HTTPS and verify certificates when talking to container registries (defaults to true).

imagebuildah/build.go

@@ -171,6 +171,8 @@ type BuildOptions struct {
 	ForceRmIntermediateCtrs bool
 	// BlobDirectory is a directory which we'll use for caching layer blobs.
 	BlobDirectory string
+	// Target the targeted FROM in the Dockerfile to build 
+	Target string
 }
 
 // Executor is a buildah-based implementation of the imagebuilder.Executor
@@ -1441,6 +1443,13 @@ func BuildDockerfiles(ctx context.Context, store storage.Store, options BuildOpt
 	if err != nil {
 		return "", nil, errors.Wrap(err, "error reading multiple stages")
 	}
+	if (options.Target != "") {
+		stagesTargeted, ok := stages.ByTarget(options.Target)
+		if !ok {
+			return "", nil, errors.Wrapf(err, "The target %q was not found in the provided Dockerfile", options.Target)
+		} 
+		stages = stagesTargeted
+	} 
 	return exec.Build(ctx, stages)
 }
 

pkg/cli/common.go

@@ -69,6 +69,7 @@ type BudResults struct {
 	SignaturePolicy     string
 	Squash              bool
 	Tag                 []string
+	Target              string
 	TlsVerify           bool
 }
 
@@ -157,6 +158,7 @@ func GetBudFlags(flags *BudResults) pflag.FlagSet {
 	fs.StringVar(&flags.SignaturePolicy, "signature-policy", "", "`pathname` of signature policy file (not usually used)")
 	fs.BoolVar(&flags.Squash, "squash", false, "Squash newly built layers into a single new layer. The build process does not currently support caching so this is a NOOP.")
 	fs.StringSliceVarP(&flags.Tag, "tag", "t", []string{}, "tagged `name` to apply to the built image")
+	fs.StringVar(&flags.Target, "target", "", "set the target build stage to build")
 	fs.BoolVar(&flags.TlsVerify, "tls-verify", true, "require HTTPS and verify certificates when accessing the registry")
 	return fs
 }

tests/bud.bats

@@ -1172,3 +1172,16 @@ load helpers
   run test -e $mnt/usr/local/bin/composer
   [ "$status" -eq 0 ]
 }
+
+@test "bud-target" {
+  target=target
+  run buildah bud --signature-policy ${TESTSDIR}/policy.json -t ${target} --target mytarget ${TESTSDIR}/bud/target
+  [ "$status" -eq 0 ]
+  cid=$(buildah from ${target})
+  root=$(buildah mount ${cid})
+  run ls ${root}/1 
+  echo "$output"
+  [ "$status" -eq 0 ]
+  buildah umount ${cid}
+  buildah rm ${cid}
+}

tests/bud/target/Dockerfile

@@ -0,0 +1,5 @@
+FROM fedora:latest
+RUN touch /foo
+
+FROM alpine:latest AS mytarget 
+RUN touch /1