Add Dockerfile

From https://github.com/znc/znc/pull/1286

Dockerfile

@@ -0,0 +1,44 @@
+FROM alpine:3.3
+
+RUN adduser -S znc && addgroup -S znc
+
+RUN mkdir -p /home/znc/src /home/znc/build
+WORKDIR /home/znc/build
+ADD . /home/znc/src
+
+ARG CONFIGUREFLAGS="--enable-perl --enable-python 3.5"
+ARG CLEANCMD="apk del build-dependencies && rm -Rf /home/znc/build /home/znc/src"
+ARG MAKEFLAGS=""
+
+RUN apk add --no-cache --virtual runtime-dependencies \
+        icu \
+        openssl \
+        boost \
+        python3 \
+        perl \
+        cyrus-sasl
+
+RUN apk add --no-cache --virtual build-dependencies \
+        build-base \
+        cmake \
+        git \
+        icu-dev \
+        openssl-dev \
+        cyrus-sasl-dev \
+        perl-dev \
+        python3-dev \
+        swig \
+        gettext-dev \
+        boost-dev \
+    && /home/znc/src/configure.sh $CONFIGUREFLAGS \
+    && make $MAKEFLAGS \
+    && make install \
+    && sh -c "$CLEANCMD"
+
+USER znc
+WORKDIR /home/znc
+VOLUME /home/znc/data
+
+EXPOSE 6667
+
+ENTRYPOINT ["/usr/local/bin/znc", "-f", "-d", "/home/znc/data"]