add support to build horizon docker images from apt stable

### What
build horizon docker image from core "stable" and horizon "stable"

### Why
this is needed in order to support the new workflow described in the
ticket

### Testing
this has been tested and works with apt "testing"

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

services/horizon/docker/Dockerfile.stable

@@ -0,0 +1,18 @@
+# install Core from apt "stable"and horizon from apt "stable" 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 stable" | 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

@@ -29,11 +29,23 @@ ifndef VERSION
 	$(error VERSION environment variable must be set. For example VERSION=2.4.1-101 )
 endif
 ifndef STELLAR_CORE_VERSION
+	$(error STELLAR_CORE_VERSION environment variable must be set. )
+else
 	$(SUDO) docker build --file ./Dockerfile.core-testing --pull $(DOCKER_OPTS) \
 	--label org.opencontainers.image.created="$(BUILD_DATE)" \
-	--build-arg VERSION=$(VERSION) -t $(TAG) .
+	--build-arg VERSION=$(VERSION) --build-arg STELLAR_CORE_VERSION=$(STELLAR_CORE_VERSION) \
+	-t $(TAG) .
+endif
+
+# build Core and Horizon from apt "stable"
+docker-build-core-stable:
+ifndef VERSION
+	$(error VERSION environment variable must be set. For example VERSION=2.4.1-101 )
+endif
+ifndef STELLAR_CORE_VERSION
+	$(error STELLAR_CORE_VERSION environment variable must be set. )
 else
-	$(SUDO) docker build --pull $(DOCKER_OPTS) \
+	$(SUDO) docker build --file ./Dockerfile.stable --pull $(DOCKER_OPTS) \
 	--label org.opencontainers.image.created="$(BUILD_DATE)" \
 	--build-arg VERSION=$(VERSION) --build-arg STELLAR_CORE_VERSION=$(STELLAR_CORE_VERSION) \
 	-t $(TAG) .