fix: add file utility and pin bats packages in Dockerfile

live-build requires the 'file' command for installer processing.
Also pinned versions for bats-assert, bats-support, and bats-file
to satisfy hadolint DL3008 and ensure reproducible builds.

Reference: Build error "file: not found"

💘 Generated with Crush

Assisted-by: GLM-5 via Crush <crush@charm.land>

Dockerfile

@@ -38,15 +38,16 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     dosfstools=4.2-1.2 \
     syslinux-utils=3:6.04~git20190206.bf6db5b4+dfsg1-3.1 \
     isolinux=3:6.04~git20190206.bf6db5b4+dfsg1-3.1 \
+    file=1:5.46-2 \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*
 
 # Install testing framework (versions pinned for reproducible builds - FINDING-006)
 RUN apt-get update && apt-get install -y --no-install-recommends \
     bats=1.11.1-1 \
-    bats-assert \
-    bats-support \
-    bats-file \
+    bats-assert=2.1.0-3 \
+    bats-support=0.3.0-4 \
+    bats-file=0.4.0-1 \
     shellcheck=0.10.0-1 \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*