Drop baked-in TLS cert, run as 1001, refuse known placeholder secrets

The dev container image generated a TLS keypair at build time and
shipped it inside the image, so every pull of the same image tag was
serving the same private key. The entrypoint also reverted to USER 0
to support a dead `createusers.txt` loop and a `chmod u+s` that was a
no-op (set on a binary owned by 1001). Net result was that any RCE
in the gateway landed as root and the wire-trust posture relied on a
shared private key.

Stop generating the cert at build time: the runtime image now carries
openssl and the entrypoint mints an ephemeral self-signed cert at
first start when no cert is mounted at the configured path. Each
container instance gets its own key. Drop USER 0 entirely; the
entrypoint runs as 1001 throughout. Prune the dead createusers loop
and the `chmod u+s`.

Separately, the README and the dev compose files publish a small set
of literal placeholder values for SessionKey, SessionEncryptionKey,
and the various Token*Key fields. Operators copy-paste these into
real deployments. Refuse to start when any of those literals appear
in the corresponding config field.

dev/docker/run.sh

@@ -1,34 +1,25 @@
 #!/bin/sh
+set -e
 
-USER=rdpgw
+cd /opt/rdpgw
 
-file="/root/createusers.txt"
-if [ -f $file ]
-  then
-    while IFS=: read -r username password is_sudo
-        do
-            echo "Username: $username, Password: **** , Sudo: $is_sudo"
-
-            if getent passwd "$username" > /dev/null 2>&1
-              then
-                echo "User Exists"
-              else
-                adduser -s /sbin/nologin "$username"
-                echo "$username:$password" | chpasswd
-            fi
-    done <"$file"
+# Generate an ephemeral self-signed cert at first start when one is not
+# already mounted/present at the configured path. Each container instance
+# gets its own key; nothing is baked into the image. This is intended for
+# the dev compose stack -- production deployments should mount a real
+# certificate, or set Tls=auto so rdpgw obtains one from Let's Encrypt.
+CERT="${RDPGW_SERVER__CERT_FILE:-/opt/rdpgw/server.pem}"
+KEY="${RDPGW_SERVER__KEY_FILE:-/opt/rdpgw/key.pem}"
+if [ ! -f "${CERT}" ] && [ ! -f "${KEY}" ]; then
+  echo "Generating ephemeral self-signed cert at ${CERT} / ${KEY} (dev only)"
+  openssl req -x509 -newkey rsa:2048 -keyout "${KEY}" -out "${CERT}" \
+    -sha256 -days 365 -nodes \
+    -subj "/CN=rdpgw-ephemeral"
 fi
 
-cd /opt/rdpgw || exit 1
-
-if [ -n "${RDPGW_SERVER__AUTHENTICATION}" ]; then
-  if [ "${RDPGW_SERVER__AUTHENTICATION}" = "local" ]; then
-    echo "Starting rdpgw-auth"
-    /opt/rdpgw/rdpgw-auth &
-  fi
+if [ "${RDPGW_SERVER__AUTHENTICATION}" = "local" ]; then
+  echo "Starting rdpgw-auth"
+  /opt/rdpgw/rdpgw-auth &
 fi
 
-# drop privileges and run the application
-su -c /opt/rdpgw/rdpgw "${USER}" -- "$@" &
-wait
-exit $?
+exec /opt/rdpgw/rdpgw "$@"