docker: Added 32bit travis builder dockerfile

contrib/Dockerfile.builder.i386

@@ -0,0 +1,34 @@
+FROM i386/ubuntu:16.04
+MAINTAINER Christian Decker <decker.christian@gmail.com>
+
+RUN echo deb http://ppa.launchpad.net/bitcoin/bitcoin/ubuntu xenial main > /etc/apt/sources.list.d/bitcoin-bitcoin-xenial.list
+RUN apt-get update -qq
+RUN apt-get install -qq -y --no-install-recommends --allow-unauthenticated \
+    asciidoc \
+    curl \
+    git \
+    make \
+    automake \
+    autoconf \
+    libtool \
+    bitcoind \
+    build-essential \
+    libprotobuf-c-dev \
+    libsodium-dev \
+    libbase58-dev \
+    libsqlite3-dev \
+    libgmp-dev \
+    libsqlite3-dev \
+    git \
+    net-tools \
+    valgrind \
+    ca-certificates \
+    python \
+    python3 \
+    python3-pip \
+    python3-setuptools \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN pip3 install python-bitcoinlib==0.7.
+RUN mkdir /build
+WORKDIR /build