Accept SOURCE_DATE_EPOCH as a build-arg

When SOURCE_DATE_EPOCH is passed in as a build-arg, treat it as we would
if it was passed in via the environment or its own CLI flag.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

imagebuildah/build.go

@@ -17,6 +17,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"time"
 
 	"github.com/containerd/platforms"
 	"github.com/containers/buildah"
@@ -219,6 +220,15 @@ func BuildDockerfiles(ctx context.Context, store storage.Store, options define.B
 		}
 	}
 
+	if sourceDateEpoch, ok := options.Args[internal.SourceDateEpochName]; ok && options.SourceDateEpoch == nil {
+		sde, err := strconv.ParseInt(sourceDateEpoch, 10, 64)
+		if err != nil {
+			return "", nil, fmt.Errorf("parsing SOURCE_DATE_EPOCH build-arg %q: %w", sourceDateEpoch, err)
+		}
+		sdeTime := time.Unix(sde, 0)
+		options.SourceDateEpoch = &sdeTime
+	}
+
 	systemContext := options.SystemContext
 	for _, platform := range options.Platforms {
 		platformContext := *systemContext

tests/bud.bats

@@ -8117,3 +8117,32 @@ _EOF
     fi
   done
 }
+
+@test "bud-with-source-date-epoch-arg" {
+  _prefetch busybox
+  local timestamp=60
+  local datestamp="1970-01-01T00:01:00Z"
+  mkdir -p $TEST_SCRATCH_DIR/buildcontext
+  cat > $TEST_SCRATCH_DIR/buildcontext/Dockerfile <<EOF
+  FROM busybox
+  RUN echo \$SOURCE_DATE_EPOCH | tee /SOURCE_DATE_EPOCH_BEFORE
+  ARG SOURCE_DATE_EPOCH
+  RUN echo \$SOURCE_DATE_EPOCH | tee /SOURCE_DATE_EPOCH_AFTER
+EOF
+  run_buildah build --layers --no-cache --build-arg=SOURCE_DATE_EPOCH=$timestamp --rewrite-timestamp -t target $TEST_SCRATCH_DIR/buildcontext
+  run_buildah from target
+  local cid="$output"
+  run_buildah run "$cid" cat /SOURCE_DATE_EPOCH_BEFORE
+  assert "$output" = "" "SOURCE_DATE_EPOCH wasn't empty before it was declared"
+  run_buildah run "$cid" cat /SOURCE_DATE_EPOCH_AFTER
+  assert "$output" = "$timestamp" "SOURCE_DATE_EPOCH was wrong after it was declared"
+  run_buildah build --layers --no-cache --build-arg=SOURCE_DATE_EPOCH=$timestamp --rewrite-timestamp -t oci:$TEST_SCRATCH_DIR/layout $TEST_SCRATCH_DIR/buildcontext
+  local manifest=${TEST_SCRATCH_DIR}/layout/$(oci_image_manifest ${TEST_SCRATCH_DIR}/layout)
+  run jq -r '.annotations."org.opencontainers.image.created"' $manifest
+  assert $status = 0 "error running jq"
+  assert "$output" = "$datestamp" "SOURCE_DATE_EPOCH build arg didn't affect image creation date annotation"
+  local config=${TEST_SCRATCH_DIR}/layout/$(oci_image_config ${TEST_SCRATCH_DIR}/layout)
+  run jq -r .created $config
+  assert $status = 0 "error running jq"
+  assert "$output" = "$datestamp" "SOURCE_DATE_EPOCH build arg didn't affect image config creation date"
+}