docker/alpine: Update buildscript to keep the docker image around

Don't delete the Alpine docker image after the build.

Also, extract the packages from the build stage, so that we can
remove them from the final image.

.dockerignore

@@ -5,3 +5,4 @@
 **/*.lo
 **/*.so
 **/.libs
+docker/alpine/pkgs

docker/alpine/Dockerfile

@@ -1,6 +1,5 @@
 # This stage builds a dist tarball from the source
 FROM alpine:edge as source-builder
-ARG commit
 
 RUN mkdir -p /src/alpine
 COPY alpine/APKBUILD.in /src/alpine
@@ -13,11 +12,12 @@ RUN source /src/alpine/APKBUILD.in \
 		gzip
 
 COPY . /src
+ARG PKGVER
 RUN cd /src \
 	&& ./bootstrap.sh \
 	&& ./configure \
 		--enable-numeric-version \
-		--with-pkg-extra-version=_git$commit \
+		--with-pkg-extra-version="_git$PKGVER" \
 	&& make dist
 
 # This stage builds an apk from the dist tarball
@@ -52,6 +52,7 @@ RUN echo 'http://dl-cdn.alpinelinux.org/alpine/edge/testing' >> /etc/apk/reposit
 		tini \
 	&& apk add \
 		--no-cache \
-		--allow-untrusted /pkgs/apk/*/*.apk
+		--allow-untrusted /pkgs/apk/*/*.apk \
+	&& rm -rf /pkgs
 COPY docker/alpine/docker-start /usr/lib/frr/docker-start
 ENTRYPOINT [ "/sbin/tini", "--", "/usr/lib/frr/docker-start" ]

docker/alpine/build.sh

@@ -1,17 +1,30 @@
 #!/bin/sh
 
 set -e
-set -v
 set -x
 
 ##
-# commit must be converted to decimal
+# Package version needs to be decimal
 ##
-c=`git rev-parse --short=10 HEAD`
-commit=`printf '%u\n' 0x$c`
-docker build -f docker/alpine/Dockerfile \
-	--build-arg commit=$commit -t frr:alpine-$c .
-id=`docker create frr:alpine-$c`
-docker cp ${id}:/pkgs/ docker/alpine
-docker rm $id
-docker rmi frr:alpine-$c
+GITREV="$(git rev-parse --short=10 HEAD)"
+PKGVER="$(printf '%u\n' 0x$GITREV)"
+
+docker build \
+	--pull \
+	--file=docker/alpine/Dockerfile \
+	--build-arg="PKGVER=$PKGVER" \
+	--tag="frr:alpine-builder-$GITREV" \
+	--target=alpine-builder \
+	.
+
+CONTAINER_ID="$(docker create "frr:alpine-builder-$GITREV")"
+docker cp "${CONTAINER_ID}:/pkgs/" docker/alpine
+docker rm "${CONTAINER_ID}"
+
+docker build \
+	--file=docker/alpine/Dockerfile \
+	--build-arg="PKGVER=$PKGVER" \
+	--tag="frr:alpine-$GITREV" \
+	.
+
+docker rmi "frr:alpine-builder-$GITREV"