Allow FROM to be overriden with build-arg

This changes will allow the "FROM" statement in a Containerfile
to be overriden with a build-arg.  If I have this Dockerfile.fedora

```
FROM fedora
```

This command will instead build an alpine image:

```
STEP 1: FROM alpine
Completed short name "alpine" with unqualified-search registries (origin: /etc/containers/registries.conf)
Getting image source signatures
Copying blob 188c0c94c7c5 done
Copying config d6e46aa247 done
Writing manifest to image destination
Storing signatures
STEP 2: COMMIT tom
--> d6e46aa2470
d6e46aa2470df1d32034c6707c8041158b652f38d2a9ae3d7ad7e7532d22ebe0
```

Addresses: containers#2404

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

docs/buildah-bud.md

@@ -61,6 +61,9 @@ instructions read from the Containerfiles in the same way that environment
 variables are, but which will not be added to environment variable list in the
 resulting image's configuration.
 
+Please refer to the [BUILD TIME VARIABLES](#build-time-variables) section for the
+list of variables that can be overridden within the Containerfile at run time.
+
 **--cache-from**
 
 Images to utilise as potential cache sources. Buildah does not currently support caching so this is a NOOP.
@@ -657,6 +660,25 @@ will convert /foo into a `shared` mount point.  The propagation properties of th
 mount can be changed directly. For instance if `/` is the source mount for
 `/foo`, then use `mount --make-shared /` to convert `/` into a `shared` mount.
 
+## BUILD TIME VARIABLES
+
+The ENV instruction in a Containerfile can be used to define variable values.  When the image
+is built, the values will persist in the container image.  At times it is more convenient to 
+change the values in the Containerfile via a command-line option rather than changing the 
+values within the Containerfile itself.
+
+The following variables can be used in conjunction with the `--build-arg` option to override the
+values set in the Containerfile.
+
+  * HTTP_PROXY
+  * HTTPS_PROXY
+  * FTP_PROXY
+  * NO_PROXY
+
+In addition, the FROM instruction can also be overridden.  Please note that if there are multiple
+FROM instructions in a Containerfile, all of the values for FROM will be overwritten throughout
+the file.  
+
 ## EXAMPLE
 
 ### Build an image using local Containerfiles
@@ -716,6 +738,15 @@ buildah bud --dns-search=example.com --dns=223.5.5.5 --dns-option=use-vc .
 
   Note: supported compression formats are 'xz', 'bzip2', 'gzip' and 'identity' (no compression).
 
+### Using Build Time Variables
+#### Replace the value for the FROM instruction in the Containerfile with `alpine`.
+
+buildah bud --build-arg=FROM=alpine .
+
+#### Replace the value set for the HTTP_PROXY environment variable within the Containerfile.
+
+buildah bud --build-arg=HTTP_PROXY="http://127.0.0.1:8321"
+
 ## ENVIRONMENT
 
 **BUILD\_REGISTRY\_SOURCES**

imagebuildah/executor.go

@@ -47,6 +47,7 @@ var builtinAllowedBuildArgs = map[string]bool{
 	"ftp_proxy":   true,
 	"NO_PROXY":    true,
 	"no_proxy":    true,
+	"FROM":        true,
 }
 
 // Executor is a buildah-based implementation of the imagebuilder.Executor
@@ -114,6 +115,7 @@ type Executor struct {
 	logRusage                      bool
 	imageInfoLock                  sync.Mutex
 	imageInfoCache                 map[string]imageTypeAndHistoryAndDiffIDs
+	fromOverride                   string
 }
 
 type imageTypeAndHistoryAndDiffIDs struct {
@@ -229,6 +231,7 @@ func NewExecutor(store storage.Store, options BuildOptions, mainNode *parser.Nod
 		jobs:                           jobs,
 		logRusage:                      options.LogRusage,
 		imageInfoCache:                 make(map[string]imageTypeAndHistoryAndDiffIDs),
+		fromOverride:                   "",
 	}
 	if exec.err == nil {
 		exec.err = os.Stderr
@@ -245,6 +248,10 @@ func NewExecutor(store storage.Store, options BuildOptions, mainNode *parser.Nod
 			fmt.Fprintf(exec.out, prefix+format+suffix, args...)
 		}
 	}
+
+	// If we have a build-arg for FROM, remember it.
+	exec.fromOverride = options.Args["FROM"]
+
 	for arg := range options.Args {
 		if _, isBuiltIn := builtinAllowedBuildArgs[arg]; !isBuiltIn {
 			exec.unusedArgs[arg] = struct{}{}
@@ -522,6 +529,9 @@ func (b *Executor) Build(ctx context.Context, stages imagebuilder.Stages) (image
 				switch strings.ToUpper(child.Value) { // first token - instruction
 				case "FROM":
 					if child.Next != nil { // second token on this line
+						if b.fromOverride != "" {
+							child.Next.Value = b.fromOverride
+						}
 						base := child.Next.Value
 						if base != "scratch" {
 							// TODO: this didn't undergo variable and arg

tests/bud.bats

@@ -2412,3 +2412,10 @@ EOF
 
   run_buildah 1 run myctr ls -l subdir/sub2.txt
 }
+
+@test "bud-caching-from-scratch-config" {
+  _prefetch alpine
+  # override the fedora image in the Containerfile
+  run_buildah bud --signature-policy ${TESTSDIR}/policy.json --build-arg=FROM=alpine ${TESTSDIR}/bud/build-arg-from
+  expect_output --substring "STEP 1: FROM alpine"
+}

tests/bud/build-arg-from/Containerfile

@@ -0,0 +1 @@
+FROM fedora