Reduce docker image for emailservice to ~240 MB (down from ~ 1.31 GB)
Main application (`email_server.py`) now runs as python 2.7. Before we had both Python 2.7 and Python 3 installed in the image.
Switched to using `python:2.7-alpine3.8` as the base image, and used multi-stage dockerfiles to keep dependencies minimal.
Fixes#49
From my shell:
```
$ docker build -t emailservice:dev . && docker run -it emailservice:dev
Sending build context to Docker daemon 97.28kB
Step 1/17 : FROM python:2.7-alpine3.8 as base
---> b2bc7255b42c
Step 2/17 : FROM base as builder
---> b2bc7255b42c
Step 3/17 : RUN apk add --update --no-cache gcc linux-headers make musl-dev python-dev g++ cairo-dev cairo openssl-dev gobject-introspection-dev
---> Using cache
---> 6daf3d9fe49a
Step 4/17 : ENV GRPC_PYTHON_VERSION 1.15.0
---> Using cache
---> 3e33d97d9580
Step 5/17 : RUN python -m pip install --upgrade pip
---> Using cache
---> e8fa3879c282
Step 6/17 : RUN pip install grpcio==${GRPC_PYTHON_VERSION} grpcio-tools==${GRPC_PYTHON_VERSION}
---> Using cache
---> c6fba7743eed
Step 7/17 : COPY requirements.txt .
---> Using cache
---> 1f6b0a444980
Step 8/17 : RUN pip install -r requirements.txt
---> Using cache
---> 8cc0a7af6aa8
Step 9/17 : FROM base as final
---> b2bc7255b42c
Step 10/17 : RUN GRPC_HEALTH_PROBE_VERSION=v0.1.0-alpha.1 && wget -qO/bin/grpc_health_probe https://github.com/grpc-ecosystem/grpc-health-probe/releases/download/${GRPC_HEALTH_PROBE_VERSION}/grpc_health_probe-linux-amd64 && chmod +x /bin/grpc_health_probe
---> Using cache
---> e954a0384081
Step 11/17 : ENV PYTHONUNBUFFERED=0
---> Using cache
---> 64ece3d72a66
Step 12/17 : WORKDIR /email_server
---> Using cache
---> 27b34dc14492
Step 13/17 : COPY --from=builder /usr/local/lib/python2.7/ /usr/local/lib/python2.7/
---> Using cache
---> 60035ec8dfd4
Step 14/17 : RUN apk add --no-cache libstdc++
---> Using cache
---> 920be90c126e
Step 15/17 : COPY . .
---> Using cache
---> 9541bed2d7a0
Step 16/17 : EXPOSE 8080
---> Using cache
---> 48fbeaa852b9
Step 17/17 : ENTRYPOINT [ "python", "email_server.py" ]
---> Using cache
---> ff317770992d
Successfully built ff317770992d
Successfully tagged emailservice:dev
starting the email service in dummy mode.
listening on port: 8080
```
