Add build command for Dockerfile

Dockerfile

+FROM alpine:latest
+    MAINTAINER Liu <liumiaogemini@foxmail.com>
+
+RUN sed -i 's/dl-cnd.alpinelinux.org/mirrors.ustc.edu.cn/g' /etc/apk/repositories && apk update \
+    && apk add git gawk vim gcc libc-dev libacl make autoconf automake pkgconf texinfo \
+    gnutls-dev ncurses-dev jansson-dev sqlite sqlite-dev dbus dbus-dev \
+    clang-extra-tools bear
+RUN git clone -b master --depth 1 git://git.sv.gnu.org/emacs.git ~/emacs
+RUN cd ~/emacs && ./autogen.sh && ./configure --without-mailutils && bear -- make && make install
+
+ADD . $HOME/.emacs.d
+
+EXPOSE 22