Update links to example docker files in the Unit in Docker section

[javorszky]

Unit's documenation on running it in Docker contains links to example documentation that uses version 1.31.1 as a base image. At the time of writing this issue the current version is 1.33.0

We should keep those examples up to date as well.

Link: https://unit.nginx.org/howto/docker/

Section:

Note

For app containerization examples, refer to our sample Go, Java, Node.js, Perl, PHP, Python, and Ruby Dockerfiles; also, see a more elaborate discussion below.

[avahahn]

Lets also make sure there is a note that points out the importance of using the right unit sub-image! (unit-wasm vs unit-python, etc).

Also, as @callahad points out, we do have a latest tag.