Add stellar-core to stellar/stellar-horizon image

Also:
* Add default versions for Horizon and Core
* Use ENTRYPOINT instead of CMD (otherwise `docker run subcommand`
  doesn't result in a horizon subcommand being run)

services/horizon/docker/Dockerfile

@@ -1,13 +1,14 @@
 FROM ubuntu:focal
 
-ARG VERSION
+ARG VERSION=2.9.0-165
+ARG CORE_VERSION=18.0.3-746.f3baea6.focal
 ARG DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update && apt-get install -y wget apt-transport-https gnupg2 && \
     wget -qO /etc/apt/trusted.gpg.d/SDF.asc https://apt.stellar.org/SDF.asc && \
     echo "deb https://apt.stellar.org focal stable" | tee -a /etc/apt/sources.list.d/SDF.list && \
-    apt-get update && apt-get install -y stellar-horizon=${VERSION} && \
+    apt-get update && apt-cache madison stellar-horizon stellar-core && apt-get install -y stellar-horizon=${VERSION} stellar-core=${CORE_VERSION} && \
     apt-get clean && rm -rf /var/lib/apt/lists/* /var/log/*.log /var/log/*/*.log
 
 EXPOSE 8000
-CMD ["/usr/bin/stellar-horizon"]
+ENTRYPOINT ["/usr/bin/stellar-horizon"]

services/horizon/docker/Dockerfile.dev

@@ -1,4 +1,4 @@
-FROM golang:1.16 AS builder
+FROM golang:1.17 AS builder
 
 WORKDIR /go/src/github.com/stellar/go
 COPY go.mod go.sum ./