I had been doing this with a multiline command on the main telegraf image but wanted to clean up my setup so created this.
This uses a modified version of a script from @Vince Loschiavo: powerwallstats.sh
At this point I've forgotten where I got the telegraf config, if/when I remember I'll update this.
POWERWALL_PASSWORD=
INFLUX_DB=
POWERWALL_IP=powerwall
COOKIE_FILE=/var/tmp/PWcookie.txt