The newest versions of docker-compose want to have some values double-quoted in the env file while old versions failing to process such files.
The solution is simple, move some of the parameters to the `docker-compose.yml` and rely on anchors for inheritance.
Since the main idea behind env files was to keep "secret" information off the main YAML we also get rid of any non-secret stuff, mainly located in the etcd.env.
The only python-etcd3 client working directly via gRPC still supports only a single endpoint, which is not very nice for high-availability.
Since Patroni is already using a heavily hacked version of python-etcd with smart retries and auto-discovery out-of-the-box, I decided to enhance the existing code with limited support of v3 protocol via gRPC-gateway.
Unfortunately, watches via gRPC-gateway requires us to open and keep the second connection to the etcd.
Known limitations:
* The very minimal supported version is 3.0.4. On earlier versions transactions don't work due to bugs in grpc-gateway. Without transactions we can't do atomic operations, i.e. leader locks.
* Watches work only starting from 3.1.0
* Authentication works only starting from 3.3.0
* gRPC-gateway does not support authentication using TLS Common Name. This is because gRPC-proxy terminates TLS from its client so all the clients share a cert of the proxy: https://github.com/etcd-io/etcd/blob/master/Documentation/op-guide/authentication.md#using-tls-common-name
1. Multi-stage build with an extensive cleanup of useless files and optional image compression
2. Start three-node etcd cluster
3. Start three-node Patroni cluster
4. One container with haproxy
5. All container names are prefixed with "demo-" and don't have suffixes
6. Decommission dev_patroni_cluster.sh script, docker-compose is now standard de-facto.
7. Provide more examples in the docker/README.md
