provider_priority added for alpine packages

As virtual packages are not versioned within a single release provider_priority
does the same thing for them as pkgver for normal packages.

alpine/APKBUILD-base.in

diff --git a/alpine/APKBUILD-base.in b/alpine/APKBUILD-base.in
index b350c18..13c1792 100644
--- a/alpine/APKBUILD-base.in
+++ b/alpine/APKBUILD-base.in
@@ -12,6 +12,7 @@
 arch="all"
 license="2-clause BSD-like license"
 provides="nginx-r$_base_version"
+provider_priority=$_base_release
 depends=""
 makedepends="linux-headers openssl-dev pcre-dev zlib-dev"