fix: use apk for Alpine-based registry image

- Replace apt-get with apk for Alpine Linux
- Simplify package installation command

Dockerfile

@@ -1,10 +1,4 @@
 FROM registry:2
 
-USER root
-# Install Apache utilities for htpasswd management
-RUN set -ex \
-    && apt-get update \
-    && apt-get install -y apache2-utils \
-    && rm -rf /var/lib/apt/lists/*
-
-USER nobody
+# The registry:2 image is based on Alpine, so we use apk
+RUN apk add --no-cache apache2-utils