Add Dockerfile to enable cross-compiling in MacOS.

In some cases for dev purposes we need to be able to produce linux compatible binary from MacOS system.
GOOS=linux is not enough witch our c-deps.

.dockerignore

@@ -0,0 +1,12 @@
+# Exclude stuff to avoid large Docker context.
+**qed
+storage
+!storage
+**riot
+**prometheus
+**/.git
+**/.terraform
+c-deps/jemalloc
+c-deps/libs
+c-deps/snappy
+c-deps/rocksdb

Dockerfile

@@ -0,0 +1,47 @@
+# Copyright 2018-2019 Banco Bilbao Vizcaya Argentaria, S.A.
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+FROM golang:1.12.1
+
+ENV GO111MODULE=on
+ENV CGO_LDFLAGS_ALLOW='.*'
+
+WORKDIR /go/src/github.com/bbva/qed
+
+# Install deps.
+RUN apt update -qq && apt install -qq -y autoconf cmake
+
+# Build C deps. 
+# This step acts as cache to avoid recompiling when Go code changes.
+RUN git clone https://github.com/BBVA/qed.git /tmp/qed  &&\
+    cd /tmp/qed                                         &&\
+    git submodule update --init --recursive             &&\
+    cd c-deps                                           &&\
+    ./builddeps.sh
+
+# Warm Go modules cache.
+RUN cd /tmp/qed      &&\
+    go mod download
+
+# Copy QED source from current working dir. 
+COPY . /go/src/github.com/bbva/qed
+
+# Move C deps to current working dir.
+RUN mv /tmp/qed/c-deps/* c-deps/
+
+# Build QED binary
+RUN go build -o /usr/local/bin/qed
+
+# Clean
+RUN rm -rf /var/lib/apt/lists/* /tmp/qed