Push and pull 'nightly' tag Docker images in nightly test job (#25619)

Adjust the nightly Docker testing script to push (and pull) `nightly`
tags of the images.

This incidentally makes the chapel-gasnet and chapel-gasnet-smp images
build off of the just-built Chapel base image. Previously it was
building off of the `latest` tag from the last release, not accessing
the local copy of `latest` due to [behavior of the buildx
builder](docker/buildx#847).

While here, adjust formatting of `util/cron/test-docker.bash` script for
clarity.

Resolves Cray/chapel-private#6436.

[reviewed by @arezaii , thanks!]

Testing:
- [x] manual run succeeds and pushes `nightly` images on chapelmac-m1
- [x] build exits with error if Chapel base image can't build
- [x] (pre-merge reminder) update Dockerhub page to note the meaning of
the `nightly` tag

util/cron/test-docker.bash

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 #
-# This script will build, run, and verify chapel, gasnet, and gasnet-smp docker images. 
+# This script will build, run, and verify chapel, gasnet, and gasnet-smp docker images.
 
 CWD=$(cd $(dirname $0) ; pwd)
 source $CWD/common.bash
@@ -9,41 +9,63 @@ export CHPL_HOME=$(cd $CWD/../.. ; pwd)
 log_info "Setting CHPL_HOME to: ${CHPL_HOME}"
 
 start_docker
- 
+
 # build_image function takes image name and docker script location as arguments.
 # Builds the image with the name from arg$1, runs the container and execute the install and verify script located in the location $2.
-build_image(){
-
-local imageName="$1"
-local script="$2" 
-# Remove any existing image with the tag before building docker image
-docker image rm --force $imageName
-docker build --load . -t $imageName
-containerid= docker image ls | grep $imageName | awk '{print$3}'
-cd ${CHPL_HOME}/util/cron
-echo 'writeln("Hello, world!");' > hello.chpl
-
-docker run --rm -i $imageName  <  $script
-
-CONTAINER_RUN=$?
-if [ $CONTAINER_RUN -ne 0 ]
-then
-      echo "docker commands failed inside chapel $imageName container" 
-      exit 1
-else
-      echo "docker commands succeeded inside chapel $imageName container"
-fi   
+build_image() {
+  local imageName="$1"
+  local script="$2"
+  # Remove any existing image with the tag before building docker image
+  docker image rm --force $imageName
+
+  docker build --push . -t $imageName
+  BUILD_RESULT=$?
+  if [ $BUILD_RESULT -ne 0 ]
+  then
+        echo "docker build failed for $imageName image"
+        exit 1
+  fi
+
+  containerid= docker image ls | grep $imageName | awk '{print$3}'
+  cd ${CHPL_HOME}/util/cron
+  echo 'writeln("Hello, world!");' > hello.chpl
+
+  docker run --rm -i $imageName  <  $script
+
+  CONTAINER_RUN=$?
+  if [ $CONTAINER_RUN -ne 0 ]
+  then
+        echo "docker commands failed inside chapel $imageName container"
+        exit 1
+  else
+        echo "docker commands succeeded inside chapel $imageName container"
+  fi
+}
 
+# Patch the Dockerfile to build FROM the nightly image instead of latest.
+# Assumes the Dockerfile is available at ./Dockerfile.
+dockerfile_nightly_patch() {
+  local nightlypatch="
+1c1
+< FROM chapel/chapel:latest
+---
+> FROM chapel/chapel:nightly
+"
+  patch ./Dockerfile << EOF
+$nightlypatch
+EOF
 }
 
-# Build chapel docker image. 
+# Build chapel Docker images
 cd $CHPL_HOME
-build_image chapel/chapel  ${CHPL_HOME}/util/cron/docker-chapel.bash
+build_image chapel/chapel:nightly  ${CHPL_HOME}/util/cron/docker-chapel.bash
 
 cd $CHPL_HOME/util/packaging/docker/gasnet
-build_image chapel/chapel_gasnet ${CHPL_HOME}/util/cron/docker-gasnet.bash
+dockerfile_nightly_patch
+build_image chapel/chapel-gasnet:nightly ${CHPL_HOME}/util/cron/docker-gasnet.bash
 
 cd $CHPL_HOME/util/packaging/docker/gasnet-smp
-build_image chapel/chapel_gasnet_smp ${CHPL_HOME}/util/cron/docker-gasnet.bash
+dockerfile_nightly_patch
+build_image chapel/chapel-gasnet-smp:nightly ${CHPL_HOME}/util/cron/docker-gasnet.bash
 
 export CHPL_NIGHTLY_TEST_CONFIG_NAME="docker"