adds an apline based dockerfile (#952)

* adds an apline based dockerfile

This PR adds a docker file which builds a testing version of `bap` and is based on
`ocaml/opam2:alpine` image.

* dependencies are installed by depext, not manually

* removed apk update

* removed env varibale

docker/alpine/Dockerfile

@@ -0,0 +1,13 @@
+FROM ocaml/opam2:alpine
+
+WORKDIR /home/opam
+
+RUN opam repo add bap git://github.com/BinaryAnalysisPlatform/opam-repository#testing \
+ && opam update \
+ && opam depext --install bap --yes \
+ && opam clean -acrs \
+ && rm -rf /home/opam/.opam/4.0[2-6] \
+ && rm -rf /home/opam/.opam/4.07/.opam-switch/sources/* \
+ && rm -rf /home/opam/opam-repository
+
+ENTRYPOINT ["opam", "config", "exec", "--"]