builder: add el-7 alias for centos-7

(cherry picked from commit 6bd3c9bb23f5dfc385a66647ab175ec2812d6617)

diff --git a/builder-support/dockerfiles/Dockerfile.target.el-7 b/builder-support/dockerfiles/Dockerfile.target.el-7
new file mode 120000 (symlink)
index 0000000..c6fc1ef
--- /dev/null
+++ b/builder-support/dockerfiles/Dockerfile.target.el-7
@@ -0,0 +1 @@
+Dockerfile.target.centos-7
\ No newline at end of file