Taiga6 docker image for raspberry pi 4

[ThorstenH]

Hi,

I wanted to test a migration to the new taiga6 (via docker) following this guide:

https://taigaio.github.io/taiga-doc/dist/upgrades-5to6.html

I am using a raspberry pi 4 8GB (great for quick evaluations).

After successfully loaded the taiga5 database dump, I wanted to migrate the database as in:

```

1.6. Migrate the database

Next step is to run Taiga6 backend so the database can be migrated to the new schema:

$ docker-compose up -d taiga-back

```

But the container "taiga-docker_taiga-back_1"  stopped with the error (found in logs):

```

standard_init_linux.go:219: exec user process caused: exec format error

```

Isn't the official taiga6 docker image a multi-arch one? The raspberry is "armv7l"...