feat: Enable reading env vars from files

test/tests/parallel/set3/container_configuration/env_vars_from_files.bats

@@ -0,0 +1,62 @@
+load "${REPOSITORY_ROOT}/test/helper/setup"
+load "${REPOSITORY_ROOT}/test/helper/common"
+
+export CONTAINER1_NAME='dms-test_env-files_success'
+export CONTAINER2_NAME='dms-test_env-files_warning'
+export CONTAINER3_NAME='dms-test_env-files_error'
+
+setup_file() {
+  export TEST__FILE
+  TEST__FILE=$(mktemp)
+  export NON_EXISTENT__FILE="/tmp/non_existent_secret"
+
+  echo 1 > "${TEST__FILE}"
+}
+
+teardown_file() {
+  rm -f "${TEST__FILE}"
+  docker rm -f "${CONTAINER1_NAME}" "${CONTAINER2_NAME}" "${CONTAINER3_NAME}"
+}
+
+@test "Environment variables are loaded from files" {
+  export CONTAINER_NAME="${CONTAINER1_NAME}"
+  local CUSTOM_SETUP_ARGUMENTS=(
+    --env ENABLE_POP3__FILE="${TEST__FILE}"
+    -v "${TEST__FILE}:${TEST__FILE}"
+  )
+  _init_with_defaults
+  _common_container_setup 'CUSTOM_SETUP_ARGUMENTS'
+  run docker logs "${CONTAINER_NAME}"
+
+  assert_success
+  assert_line --partial "Getting secret ENABLE_POP3 from ${TEST__FILE}"
+  _exec_in_container [ -f /etc/dovecot/protocols.d/pop3d.protocol ]
+  assert_success
+}
+
+@test "Existing environment variables take precedence over __FILE variants" {
+  export CONTAINER_NAME="${CONTAINER2_NAME}"
+  local CUSTOM_SETUP_ARGUMENTS=(
+    --env TEST="manual-secret"
+    --env TEST__FILE="${TEST__FILE}"
+  )
+  _init_with_defaults
+  _common_container_setup 'CUSTOM_SETUP_ARGUMENTS'
+  run docker logs "${CONTAINER_NAME}"
+
+  assert_success
+  assert_line --partial "Ignoring TEST since TEST__FILE is also set"
+}
+
+@test "Non-existent file triggers an error" {
+  export CONTAINER_NAME="${CONTAINER3_NAME}"
+  local CUSTOM_SETUP_ARGUMENTS=(
+    --env TEST__FILE="${NON_EXISTENT__FILE}"
+  )
+  _init_with_defaults
+  _common_container_setup 'CUSTOM_SETUP_ARGUMENTS'
+  run docker logs "${CONTAINER_NAME}"
+
+  assert_success
+  assert_line --partial "File ${NON_EXISTENT__FILE} does not exist"
+}