Revise approach, teardown temp directory on pre-exit

This also fixes a shellcheck error SC2155
Instead of setting DOCKER_CONFIG directly, set
BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY and hydrate DOCKER_CONFIG for
plugin steps. if this gets overriden by the user, they can manage their
own DOCKER_CONFIG cleanup activities and we will always remove the
temporary directory from the agent instance filesystem.

Author: Chloe Hutchinson

packer/linux/conf/buildkite-agent/hooks/environment

@@ -5,7 +5,9 @@ set -eu -o pipefail
 source ~/cfn-env
 
 # a clean docker config for each job, for improved isolation
-export DOCKER_CONFIG=$(mktemp -d)
+BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY=$(mktemp -d)
+export BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY
+export DOCKER_CONFIG="$BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY"
 
 echo "~~~ :llama: Setting up elastic stack environment ($BUILDKITE_STACK_VERSION)"
 cat ~/cfn-env

packer/linux/conf/buildkite-agent/hooks/pre-exit

@@ -2,6 +2,11 @@
 
 set -eu -o pipefail
 
+# clean up our temporary docker config
+if [[ -n "${BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY:-}" ]]; then
+  rmdir ${BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY}
+fi
+
 if [[ -n "${BUILDKITE_SECRETS_BUCKET:-}" &&  "${SECRETS_PLUGIN_ENABLED:-}" == "1" ]] ; then
   export BUILDKITE_PLUGIN_S3_SECRETS_BUCKET="$BUILDKITE_SECRETS_BUCKET"
 

packer/windows/conf/buildkite-agent/hooks/environment

@@ -5,7 +5,9 @@ set -eu -o pipefail
 source ~/cfn-env
 
 # a clean docker config for each job, for improved isolation
-export DOCKER_CONFIG=$(mktemp -d)
+BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY=$(mktemp -d)
+export BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY
+export DOCKER_CONFIG="$BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY"
 
 echo "~~~ :llama: Setting up elastic stack environment ($BUILDKITE_STACK_VERSION)"
 cat ~/cfn-env

packer/windows/conf/buildkite-agent/hooks/pre-exit

@@ -2,6 +2,11 @@
 
 set -eu -o pipefail
 
+# clean up our temporary docker config
+if [[ -n "${BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY:-}" ]]; then
+  rmdir ${BUILDKITE_DOCKER_CONFIG_TEMP_DIRECTORY}
+fi
+
 if [[ -n "${BUILDKITE_SECRETS_BUCKET:-}" &&  "${SECRETS_PLUGIN_ENABLED:-}" == "1" ]] ; then
   export BUILDKITE_PLUGIN_S3_SECRETS_BUCKET="$BUILDKITE_SECRETS_BUCKET"