npm: interim net-tools image (thin fork of jc21 2.13.5) for docker exec diagnostics

apps/npm/build/Dockerfile

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+# INTERIM image for npm:runtipi.
+#
+# Thin fork: upstream jc21/nginx-proxy-manager (Debian bookworm) with network
+# diagnostic tools baked in, so `docker exec npm_runtipi-npm-1 ping|nslookup|
+# dig|traceroute|...` works and SURVIVES container recreation on Runtipi
+# restart/update (an in-container `apt install` does not persist).
+#
+# Stop-gap while the from-source build of the Gitea fork
+# (git.alexzaw.dev/alexz/nginx-proxy-manager) is set up. NPM itself is
+# byte-for-byte upstream here — only diagnostic packages are added, the
+# s6 entrypoint/behaviour is untouched.
+FROM jc21/nginx-proxy-manager:2.13.5
+
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends \
+      iputils-ping dnsutils traceroute mtr-tiny netcat-openbsd \
+      iproute2 net-tools tcpdump telnet whois nmap wget \
+ && apt-get clean \
+ && rm -rf /var/lib/apt/lists/*