One app I work with has 40 top level gems and about 17,000 lines of Ruby code (which goes a longs ways in Rails). It's not "big big" but it's not tiny. It takes under 10s to boot up. I'm sure if you have a Shopify-tier app it takes much longer, but a majority of apps aren't at that scale.

I was just throwing out examples and I'm just trying to say not everyone is dealing with cases where Docker's added 2-5+ seconds isn't a big deal. For a Flask app those few seconds make it ~20-50x longer to start up due to Docker and for a majority of other frameworks / average app sizes, it's a non-ignorable amount.