Each update of minidlna overwrites the systemd unit file.

/etc/systemd/system/multi-user.target.wants/minidlna.service contains:

---code---
[Service]
Type=forking
PIDFile=/var/run/minidlnad.pid
ExecStartPre=/usr/sbin/minidlnaconfig
EnvironmentFile=-/var/lib/minidlna/rescan.conf
ExecStart=/usr/sbin/minidlnad $RESCAN -P /var/run/minidlnad.pid -f
/var/lib/minidlna/minidlna.conf
---code---

/var/lib/minidlna/minidlna.conf contains:

---code---
port=8200
network_interface=eth0
user=root
media_dir=/mnt
merge_media_dirs=no
friendly_name=MiniDLNA
log_level=general,artwork,database,inotify,scanner,metadata,http,ssdp,tivo=warn
album_art_names=Cover.jpg/cover.jpg/AlbumArtSmall.jpg/albumartsmall.jpg/AlbumArt.jpg/albumart.jpg/Album.jpg/album.jpg/Folder.jpg/folder.jpg/Thumb.jpg/thumb.jpg
inotify=yes
enable_tivo=no
tivo_discovery=bonjour
strict_dlna=no
notify_interval=900
serial=12345678
model_number=1
root_container=.
wide_links=no
db_dir=/var/cache/minidlna
log_dir=/var/log/minidlna
---code---

which forces the media_dir=/mnt

I keep my music in ~/music and my minidlna.conf in /etc.
Every time I update minidlna I have to edit the Unit file to contain:

---code---
ExecStart=/usr/sbin/minidlnad $RESCAN -P /var/run/minidlnad.pid -f
/etc/minidlna.conf
---code---

followed by:

# systemctl daemon-reload
# systemctl restart minidlna