Use go1.16 "embed" to set version from VERSION file

This allows building runc without passing the VERSION build-arg, but requires
go 1.16 or up.

Unfortunately, there's no easy way to get the git-commit from the filesystem
(possibly through `.git/logs/HEAD`, which is a large file); a proposal has been
accepted to add git information ([1]), which will be included in go1.18.

For users building through `go install github.com/opencontainers/runc@version`),
we may be able to use runtime/debug.BuildInfo() ([2]). BuildInfo provides the
module version and checksum, but only when building using `go install`. When
building from a local module (git clone), the version is alway set to `(devel)`,
see [3].

We could consider add module (optional) if available, e.g.:

    // Print module information if available. When built from local source,
    // (using git clone), module version is not available, and version is
    // set to "(devel)"; see golang/go#29814, and golang/go#37475.
    if bi, ok := debug.ReadBuildInfo(); ok && bi.Main.Version != "(devel)" {
        v = append(v, "module version: "+bi.Main.Version+", sum: "+bi.Main.Sum)
    }

With this patch (on go1.16):

    make
    go build -trimpath "-buildmode=pie"  -tags "seccomp" -ldflags "-X main.gitCommit=v1.0.0-133-g27d75cca " -o runc .

    ./runc --version
    runc version 1.0.0+dev

    commit: v1.0.0-133-g27d75cca
    spec: 1.0.2-dev
    go: go1.16.7
    libseccomp: 2.3.3

[1]: golang/go#37475
[2]: https://pkg.go.dev/runtime/debug#BuildInfo
[3]: golang/go#29814

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Makefile

@@ -21,9 +21,9 @@ ifeq ($(shell $(GO) env GOOS),linux)
 	endif
 endif
 GO_BUILD := $(GO) build -trimpath $(GO_BUILDMODE) $(EXTRA_FLAGS) -tags "$(BUILDTAGS)" \
-	-ldflags "-X main.gitCommit=$(COMMIT) -X main.version=$(VERSION) $(EXTRA_LDFLAGS)"
+	-ldflags "-X main.gitCommit=$(COMMIT) $(EXTRA_LDFLAGS)"
 GO_BUILD_STATIC := CGO_ENABLED=1 $(GO) build -trimpath $(EXTRA_FLAGS) -tags "$(BUILDTAGS) netgo osusergo" \
-	-ldflags "-extldflags -static -X main.gitCommit=$(COMMIT) -X main.version=$(VERSION) $(EXTRA_LDFLAGS)"
+	-ldflags "-extldflags -static -X main.gitCommit=$(COMMIT) $(EXTRA_LDFLAGS)"
 
 .DEFAULT: runc
 

main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	_ "embed"
 	"errors"
 	"fmt"
 	"io"
@@ -17,9 +18,9 @@ import (
 	"github.com/urfave/cli"
 )
 
-// version must be set from the contents of VERSION file by go build's
-// -X main.version= option in the Makefile.
-var version = "unknown"
+// version is automatically set from the contents of VERSION file on go1.16+.
+//go:embed VERSION
+var version string
 
 // gitCommit will be the hash that the binary was built from
 // and will be populated by the Makefile