Makefile: Add EXTRA_VERSION

Add this new make variable so users can specify build information
without modifying the runc version nor the source code.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>
(cherry picked from commit cc2078c)

Makefile

@@ -12,7 +12,8 @@ PROJECT := github.com/opencontainers/runc
 BUILDTAGS ?= seccomp
 
 COMMIT ?= $(shell git describe --dirty --long --always)
-VERSION := $(shell cat ./VERSION)
+EXTRA_VERSION :=
+VERSION := $(shell cat ./VERSION)$(EXTRA_VERSION)
 LDFLAGS_COMMON := -X main.gitCommit=$(COMMIT) -X main.version=$(VERSION)
 
 GOARCH := $(shell $(GO) env GOARCH)

README.md

@@ -56,6 +56,16 @@ sudo make install
 
 `runc` will be installed to `/usr/local/sbin/runc` on your system.
 
+#### Version string customization
+
+You can see the runc version by running `runc --version`. You can append a custom string to the
+version using the `EXTRA_VERSION` make variable when building, e.g.:
+
+```bash
+make EXTRA_VERSION="+build-1"
+```
+
+Bear in mind to include some separator for readability.
 
 #### Build Tags