docker: Start kmd in the background.

[winder]

Summary

The user is left out intentionally, see #5464

This doesn't fix the root issue where it takes 30 seconds for kmd to start, but should allow it to startup without delay.

Test Plan

Test against rel/stable and this PR branch:

docker build \
             -t wwinder/algod:exec \
             --build-arg CHANNEL=stable \
             --build-arg TARGETARCH=amd64 --no-cache \
             .

docker run --rm -it \
             -e START_KMD=1 \
             --name algod-test-run \
             -v (pwd)/docker_data_dir:/algod/data \
             wwinder/algod:exec

on rel/stable it takes 30 seconds to startup due to kmd locking the pidfile, which is due to the user being managed incorrectly with exec and runuser.

With this branch it starts in less than 5 seconds.

[codecov]

Codecov Report

Merging #5514 (1606372) into master (68ebeb5) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5514      +/-   ##
==========================================
- Coverage   55.78%   55.77%   -0.02%     
==========================================
  Files         446      446              
  Lines       63228    63228              
==========================================
- Hits        35271    35264       -7     
- Misses      25585    25596      +11     
+ Partials     2372     2368       -4     

see 20 files with indirect coverage changes

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[algochoi]

Runs as expected and starts up without delay on my machine 👍