Configure from env vars at run time

In both local developent, and production.  Also, vendor the `envsubst`
binary from https://github.com/a8m/envsubst, as it supports default
values, which the gnu envsubst (from gettext-base) does not.

services/git-bridge/Dockerfile

@@ -2,16 +2,20 @@
 
 FROM maven:3-jdk-8 as base
 
-RUN apt-get update && apt-get install -y make \
+RUN apt-get update && apt-get install -y make git \
  && rm -rf /var/lib/apt/lists
 
+COPY vendor/envsubst /opt/envsubst
+RUN chmod +x /opt/envsubst
+
 RUN useradd --create-home node
 
 FROM base as builder
 
+COPY . /app
+
 WORKDIR /app
 
-COPY . /app
 RUN make package \
 # The name of the created jar contains the current version tag.
 # Rename it to a static path that can be used for copying.
@@ -21,15 +25,16 @@ RUN make package \
 
 FROM openjdk:8-jre
 
-RUN apt-get update && apt-get install -y git gettext-base\
+RUN apt-get update && apt-get install -y git \
  && rm -rf /var/lib/apt/lists
 
 RUN useradd --create-home node
 
-CMD ["/start.sh"]
-
 COPY --from=builder /git-bridge.jar /
 
+COPY vendor/envsubst /opt/envsubst
+RUN chmod +x /opt/envsubst
+
 COPY conf/envsubst_template.json envsubst_template.json
 COPY start.sh start.sh
 
@@ -37,3 +42,5 @@ RUN mkdir conf
 RUN chown node:node conf
 
 USER node
+
+ENTRYPOINT ["/start.sh"]