Provide stolonctl environment variables to shells (#79)

Fixes #13.  We want to set the environment variables
stolonctl expects (STOLONCTL_CLUSTER_NAME, etc.) on shells
so users can fly ssh console in and run stolonctl to more
easily administrate their cluster.

Those environment variables are written to a file at
/data/.env by the "start" Go program after the defaults
are set.

Unfortunately, the shell that is created by hallpass appears to be
hard coded to /bin/sh, is not a login shell nor controllable by
/etc/passwd, so our best option is to use the POSIX ENV variable.

Dockerfile

@@ -56,6 +56,8 @@ RUN useradd -ms /bin/bash stolon
 RUN mkdir -p /run/haproxy/
 COPY --from=flyutil /fly/bin/* /usr/local/bin/
 
+ENV ENV="/fly/set-stolon-environment.sh"
+
 EXPOSE 5432
 
 CMD ["start"]

scripts/set-stolon-environment.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+if [ -f /data/.env ]; then
+  export $(cat /data/.env)
+fi