dockerfile and env var

Add Dockerfile, requirement.txt, and new BUMPER_CERTS env var.

Dockerfile

@@ -0,0 +1,19 @@
+FROM python:3.7-alpine as base
+
+FROM base as builder
+
+RUN mkdir /install
+WORKDIR /install
+
+COPY requirements.txt /requirements.txt
+
+RUN pip install --install-option="--prefix=/install" -r /requirements.txt
+
+FROM base
+
+COPY --from=builder /install /usr/local
+COPY . /bumper
+
+WORKDIR /bumper
+
+ENTRYPOINT ["python3", "-m", "bumper"]