Limit _FILE env var support to specific vars

Closes elastic#52503. Implement a list of `_FILE` env vars that will be used to
populate env vars with file content, instead of processing all `_FILE`
vars in the environment.

distribution/src/bin/elasticsearch-env-from-file

@@ -7,11 +7,15 @@ set -e -o pipefail
 # point to it. This can be used to provide secrets to a container, without
 # the values being specified explicitly when running the container.
 #
+# Note that only supported environment variables are processed, in order
+# to avoid unexpected failures when an environment sets a "*_FILE" variable
+# that doesn't contain a filename.
+#
 # This script is intended to be sourced, not executed, and modifies the
 # environment.
 
-for VAR_NAME_FILE in $(env | cut -f1 -d= | grep '_FILE$'); do
-  if [[ -n "$VAR_NAME_FILE" ]]; then
+for VAR_NAME_FILE in ELASTIC_PASSWORD_FILE KEYSTORE_PASSWORD_FILE ES_JAVA_OPTS_FILE; do
+  if [[ -n "${!VAR_NAME_FILE}" ]]; then
     VAR_NAME="${VAR_NAME_FILE%_FILE}"
 
     if env | grep "^${VAR_NAME}="; then