use exec form for Dockerfile entrypoint

This causes the jellyfin process to be pid 1 in the container.  It can
receive signals like SIGTERM (from "docker stop", for example) and shut
down properly.

Dockerfile

@@ -42,7 +42,7 @@ ENV DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=1
 
 EXPOSE 8096
 VOLUME /cache /config /media
-ENTRYPOINT ./jellyfin/jellyfin \
-    --datadir /config \
-    --cachedir /cache \
-    --ffmpeg /usr/local/bin/ffmpeg
+ENTRYPOINT ["./jellyfin/jellyfin", \
+    "--datadir", "/config", \
+    "--cachedir", "/cache", \
+    "--ffmpeg", "/usr/local/bin/ffmpeg"]

Dockerfile.arm

@@ -38,7 +38,7 @@ ENV DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=1
 
 EXPOSE 8096
 VOLUME /cache /config /media
-ENTRYPOINT ./jellyfin/jellyfin \
-    --datadir /config \
-    --cachedir /cache \
-    --ffmpeg /usr/bin/ffmpeg
+ENTRYPOINT ["./jellyfin/jellyfin", \
+    "--datadir", "/config", \
+    "--cachedir", "/cache", \
+    "--ffmpeg", "/usr/bin/ffmpeg"]

Dockerfile.arm64

@@ -38,7 +38,7 @@ ENV DOTNET_SYSTEM_GLOBALIZATION_INVARIANT=1
 
 EXPOSE 8096
 VOLUME /cache /config /media
-ENTRYPOINT ./jellyfin/jellyfin \
-    --datadir /config \
-    --cachedir /cache \
-    --ffmpeg /usr/bin/ffmpeg
+ENTRYPOINT ["./jellyfin/jellyfin", \
+    "--datadir", "/config", \
+    "--cachedir", "/cache", \
+    "--ffmpeg", "/usr/bin/ffmpeg"]