fix: support Adminer 6, remove custom Docker build

[stasadev]

The Issue

Adminer 6 accepts a login only with a valid CSRF token and a same-site request, so the forged login POST stopped working and the login form was served instead. No issue was filed, it showed up in the tests: https://github.com/ddev/ddev-adminer/actions/runs/32349923909

not ok 1 install from directory
# (from function `__assert_stream' in file /home/linuxbrew/.linuxbrew/lib/bats-assert/src/assert_output.bash, line 236,
#  from function `assert_output' in file /home/linuxbrew/.linuxbrew/lib/bats-assert/src/assert_output.bash, line 125,
#  from function `health_checks' in file tests/test.bats, line 49,
#  in test file tests/test.bats, line 93)
#   `health_checks' failed
#
# -- output does not contain substring --
# substring (1 lines):
#   HTTP/2 302
# output (15 lines):
#   HTTP/2 200 
#   cache-control: no-cache
#   content-security-policy: script-src 'self' 'unsafe-inline' 'nonce-MWY2NjFmMzFjNzE3Mjc4MjBhNTdkOWRmMzIyZDM5NTE=' 'strict-dynamic'; connect-src 'self' https://www.adminer.org; frame-src https://www.adminer.org; object-src 'none'; base-uri 'none'; form-action 'self'
#   content-type: text/html; charset=utf-8
#   date: Thu, 20 Aug 2026 08:43:01 GMT
#   host: test-ddev-adminer.ddev.site:9101
#   referrer-policy: origin-when-cross-origin
#   set-cookie: adminer_sid=f55e35f0751e73de961521d5e3834374; path=/; HttpOnly; SameSite=lax
#   set-cookie: adminer_key=73c2c7d333b92123aaf21d6922893b2d; path=/; HttpOnly; SameSite=lax
#   vary: Accept-Encoding
#   x-content-type-options: nosniff
#   x-frame-options: deny
#   x-powered-by: PHP/8.4.24
#   x-xss-protection: 0
#   
# --

How This PR Solves The Issue

Log in without the form: put the password where Adminer keeps it for a logged in user and redirect to the URL holding the other credentials. That is all the plugin does now, so ddev-adminer.php and the command override are gone. entrypoint.sh always enables it, ADMINER_PLUGINS is only for the plugins you add yourself.

The custom Docker build only created a user matching the host uid, which entrypoint.sh now does, so the image is used as is, pinned to adminer:6-standalone.

Manual Testing Instructions

ddev add-on get ddev/ddev-adminer --pr 54
ddev restart

Automated Testing Overview

health_checks() asserts the database page instead of the login form, that Adminer serves as the host user, and that the login is enabled once next to the plugins from ADMINER_PLUGINS. The SQLite test writes a row through Adminer, which only succeeds while the project file stays writable for it.

Release/Deployment Notes

ADMINER_PLUGINS no longer needs ddev-passwordless-login, a duplicate from an older config is ignored. SQLite needs only the driver and the database, the unused credentials no longer have to be emptied.