udp Dockerfile: use here-doc for entrypoint creation

docker/aquatic_udp.Dockerfile

@@ -1,8 +1,13 @@
+# syntax=docker/dockerfile:1
+
 # aquatic_udp
 #
 # Customize by setting CONFIG_FILE_CONTENTS and
 # ACCESS_LIST_CONTENTS environment variables.
 #
+# By default runs tracker on port 3000 without info hash access control.
+#
+# Run from repository root directory with:
 # $ docker build -t aquatic-udp -f docker/aquatic_udp.Dockerfile .
 # $ docker run -it -p 0.0.0.0:3000:3000/udp --name aquatic-udp aquatic-udp
 
@@ -23,7 +28,15 @@ WORKDIR /root/
 
 COPY --from=builder /usr/src/aquatic/target/release/aquatic_udp ./
 
-# Setting config and access list file contents at runtime
+# Create entry point script for setting config and access
-RUN echo "#!/bin/sh\necho \"\$CONFIG_FILE_CONTENTS\" > ./config.toml\necho \"\$ACCESS_LIST_CONTENTS\" > ./access-list.txt\n./aquatic_udp -c ./config.toml" > entrypoint.sh && chmod +x entrypoint.sh
+# list file contents at runtime
+COPY <<-"EOT" ./entrypoint.sh
+#!/bin/sh
+echo "$CONFIG_FILE_CONTENTS" > ./config.toml
+echo "$ACCESS_LIST_CONTENTS" > ./access-list.txt
+exec ./aquatic_udp -P -c ./config.toml
+EOT
+
+RUN chmod +x ./entrypoint.sh
 
 ENTRYPOINT ["./entrypoint.sh"]