allow for the bundling of Core package from apt "testing" pool

### What
a new Dockerfile to install Core from the "testing" pool
an update to the Makefile specifically for this case

### Why
adding support for the "testing" pool to the Dockerfile would make
the file more confusing to read, so I decided to use a separate Dockerfile

### Testing
I will test this manually shortly

### Issue addressed by this PR
stellar/ops#3124

services/horizon/docker/Dockerfile

@@ -1,3 +1,4 @@
+# install Core from apt "stable" or "unstable" pool, and horizon from apt "testing" pool
 FROM ubuntu:focal
 
 ARG VERSION

services/horizon/docker/Dockerfile.core-testing

@@ -0,0 +1,19 @@
+# install both horizon and core from apt "testing" pool
+FROM ubuntu:focal
+
+ARG VERSION
+ARG STELLAR_CORE_VERSION
+ARG DEBIAN_FRONTEND=noninteractive
+ARG ALLOW_CORE_UNSTABLE=no
+
+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 testing" | tee -a /etc/apt/sources.list.d/SDF.list && \
+    cat /etc/apt/sources.list.d/SDF.list && \
+    apt-get update && \
+    apt-cache madison stellar-core && eval "apt-get install -y stellar-core${STELLAR_CORE_VERSION+=$STELLAR_CORE_VERSION}" && \
+    apt-cache madison stellar-horizon && apt-get install -y stellar-horizon=${VERSION} && \
+    apt-get clean && rm -rf /var/lib/apt/lists/* /var/log/*.log /var/log/*/*.log
+
+EXPOSE 8000
+ENTRYPOINT ["/usr/bin/stellar-horizon"]

services/horizon/docker/Makefile

@@ -5,6 +5,7 @@ BUILD_DATE := $(shell date -u +%FT%TZ)
 
 TAG ?= stellar/stellar-horizon:$(VERSION)
 
+# build with Core from apt "stable" or "unstable", and horizon from apt "testing"
 docker-build:
 ifndef VERSION
 	$(error VERSION environment variable must be set. For example VERSION=2.4.1-101 )
@@ -22,6 +23,22 @@ else
 	-t $(TAG) .
 endif
 
+# build Core and Horizon from apt "testing"
+docker-build-core-testing:
+ifndef VERSION
+	$(error VERSION environment variable must be set. For example VERSION=2.4.1-101 )
+endif
+ifndef STELLAR_CORE_VERSION
+	$(SUDO) docker build --file ./Dockerfile.core-testing --pull $(DOCKER_OPTS) \
+	--label org.opencontainers.image.created="$(BUILD_DATE)" \
+	--build-arg VERSION=$(VERSION) -t $(TAG) .
+else
+	$(SUDO) docker build --pull $(DOCKER_OPTS) \
+	--label org.opencontainers.image.created="$(BUILD_DATE)" \
+	--build-arg VERSION=$(VERSION) --build-arg STELLAR_CORE_VERSION=$(STELLAR_CORE_VERSION) \
+	-t $(TAG) .
+endif
+
 docker-push:
 ifndef TAG
 	$(error Must set VERSION or TAG environment variable. For example VERSION=2.4.1-101 )