Update docker image to expose `meilisearch` as the entry point rather than `tini`

Currently, Meilisearch exposes tini as the dockerfile entry point.

This creates a rather cryptic error when you try to run meili like normal e.g.

& docker run --name meilisearch -p 7700:7700 getmeili/meilisearch --master-key="foobar"

Produces:

[FATAL tini (7)] exec --master-key=foobar failed: No such file or directory

Which isn't the most helpful error in the world.

Either that or the documentation probably want's to be changed slightly to make it more clear that you need to explicitly re-run meili as:

& docker run --name meilisearch -p 7700:7700 getmeili/meilisearch ./meilisearch --master-key="foobar"

But IMO

getmeili/meilisearch ./meilisearch --master-key="foobar"

Doesn't seem to be the best situation.