Pass stack version as argument to image fetch function

This script reads and modifies environment variables for the processes it
controls. This commit changes the way the `pull_docker_snapshot` function
references variables. Specifically, we now explicitly pass arguments that are
*not* managed as environment variables by the script as explicit parameters to
the script instead of looking them up from global scope. The pattern this
attempts to acheive is that the only thing looked up from global scope in a
function is an argument that is actively being managed by the environment such
that it remains consistent across any sub processes spawned by the shell script.

Author: donoghuc

docker-setup.sh

@@ -6,12 +6,13 @@ set -e
 
 pull_docker_snapshot() {
   project="${1?project name required}"
+  stack_version="${2?stack version required}"
   local docker_image="docker.elastic.co/${project}/${project}${DISTRIBUTION_SUFFIX}:${ELASTIC_STACK_VERSION}"
   echo "Pulling $docker_image"
   if docker pull "$docker_image" ; then
     echo "docker pull successful"
   else
-    case "$ELASTIC_STACK_VERSION_ARG" in
+    case $stack_version in
       "8.previous"|"8.current"|"8.next")
         exit 1
         ;;
@@ -30,7 +31,7 @@ if [ -z "${ELASTIC_STACK_VERSION}" ]; then
     exit 1
 fi
 
-# save the original arg if needed
+# save the original unmodified string to pass to pull_docker_snapshot
 ELASTIC_STACK_VERSION_ARG="$ELASTIC_STACK_VERSION"
 
 echo "Fetching versions from $VERSION_URL"
@@ -63,7 +64,7 @@ export DISTRIBUTION_SUFFIX
 echo "Testing against version: $ELASTIC_STACK_VERSION (distribution: ${DISTRIBUTION:-"default"})"
 
 if [[ "$ELASTIC_STACK_VERSION" = *"-SNAPSHOT" ]]; then
-    pull_docker_snapshot "logstash"
+    pull_docker_snapshot "logstash" $ELASTIC_STACK_VERSION_ARG
 fi
 
 if [ -f Gemfile.lock ]; then