sec(iac/build): close the shell-injection surface on docker buildx build

terraform/environments/azure/build.tf

@@ -21,7 +21,9 @@ module "build" {
   # platform not set — auto-detected from builder host (Container Apps and AKS support arm64 and amd64)
 
   # Registry login with the caller's Entra identity (deploy SP in CI, az login
-  # locally); the principal needs AcrPush on the registry.
+  # locally); the principal needs AcrPush on the registry. Identity-based, so
+  # it carries no static credential, matching the module's
+  # registry_login_command contract (see its variable description).
   registry_login_command = "az acr login --name ${azurerm_container_registry.main.name}"
 
   # Build options

terraform/modules/build/main.tf

@@ -68,7 +68,20 @@ resource "terraform_data" "docker_build" {
 
   provisioner "local-exec" {
     working_dir = var.source_path
-    command     = <<-EOT
+    # extra_build_args is passed through the environment, not interpolated into
+    # the script text below, so shell metacharacters in its value (;, `, $())
+    # are never parsed as script syntax. registry_login_command still has to be
+    # interpolated as literal shell source (it is documented to be a full
+    # command, e.g. a pipe into `docker login`), so it stays a trusted,
+    # identity-based-only input (see its variable description) rather than a
+    # secret: neither variable is marked sensitive, because that would
+    # suppress this resource's entire local-exec output (the build/push log
+    # deploy-*.yml workflows tee and grep to detect failures), not just the
+    # two variables' own values.
+    environment = {
+      EXTRA_BUILD_ARGS = var.extra_build_args
+    }
+    command = <<-EOT
       set -e
       echo "Logging in to registry..."
       ${var.registry_login_command}
@@ -105,11 +118,11 @@ resource "terraform_data" "docker_build" {
         $PLATFORM_ARG \
         --provenance=false \
         --sbom=false \
-        --tag ${local.image_uri} \
-        --build-arg GIT_COMMIT=${local.git_commit} \
-        --build-arg BUILD_DATE=${local.timestamp} \
+        --tag "${local.image_uri}" \
+        --build-arg "GIT_COMMIT=${local.git_commit}" \
+        --build-arg "BUILD_DATE=${local.timestamp}" \
         --push \
-        ${var.extra_build_args} \
+        $EXTRA_BUILD_ARGS \
         .
 
       echo "Docker image built and pushed successfully"

terraform/modules/build/variables.tf

@@ -35,13 +35,13 @@ variable "skip_docker_build" {
 }
 
 variable "extra_build_args" {
-  description = "Extra arguments to pass to docker build"
+  description = "Extra arguments to pass to docker build. Passed through the local-exec provisioner's environment (not interpolated into the script), then whitespace-split unquoted (no shell quoting is honored): a value like \"--build-arg LABEL=hello world\" splits into two docker buildx arguments, not one. No caller sets this today; if a value ever needs an embedded space, extend the module to accept a list(string) instead."
   type        = string
   default     = ""
 }
 
 variable "registry_login_command" {
-  description = "Command to authenticate with registry (e.g., aws ecr get-login-password | docker login...)"
+  description = "Command to authenticate with registry (e.g., az acr login --name ..., aws ecr get-login-password | docker login ..., gcloud auth configure-docker ...). Runs verbatim as shell input. Must be an identity-based login (the CLI resolves/mints the credential itself); never embed a static, long-lived credential literal (a password, API key, or JSON key) in this value; it is not redacted in terraform plan/apply output, and marking it sensitive would suppress this resource's entire local-exec log (build/push progress, docker error output), which deploy-*.yml workflows tee and grep to detect build failures."
   type        = string
 }