Get rid of README and Dockerfile for Alpine Linux

As there is now support for Alpine Linux in packpack, there is no
longer any need in a custom Dockerfile builder.

apk/Dockerfile

-FROM alpine:3.5
-
-RUN apk update \
-	&& apk add alpine-sdk \
-	&& adduser -G abuild -g "Alpine Package Builder" -s /bin/sh -D builder \
-	&& echo "builder ALL=(ALL) NOPASSWD:ALL" >> /etc/sudoers
-
-USER builder
-
-RUN sudo mkdir -p /var/cache/distfiles \
-	&& sudo chmod a+w /var/cache/distfiles \
-	&& sudo chgrp abuild /var/cache/distfiles \
-	&& sudo chmod g+w /var/cache/distfiles \
-	&& abuild-keygen -a -i
-
-COPY --chown=builder:abuild . /home/builder/tarantool/
-
-WORKDIR /home/builder/tarantool/apkbuild
-
-VOLUME ["/target"]
-
-CMD ["abuild", "-r", "-P", "/target"]
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apk/README.md

-Building apk package
----
-
-```
-git clone https://github.com/tarantool/tarantool.git
-docker build -t apkbuild -f apkbuild/Dockerfile .
-docker run --rm -it -v $(pwd)/target:/target apkbuild:latest
-```
-
-Then you will find newly created package under `./target/tarantool/<arch>` dir
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