Fix ARM container builds

[perfectra1n]

There are quite a few bugs when using GitHub Actions along with node:alpine and armv7/armv6 builds. This includes the following issues:
docker/build-push-action#1071
docker/build-push-action#977
nodejs/docker-node#2077

I also tried some of the workarounds listed in the above issues, but instead got really strange errors:

docker run testingnotes
No USER_UID specified, leaving 1000
No USER_GID specified, leaving 1000
(node:1) ExperimentalWarning: Importing JSON modules is an experimental feature and might change at any time
(Use `node --trace-warnings ...` to show where the warning was created)
Generated session secret


#
# Fatal error in , line 0
# Check failed: module->status() == kLinked || module->status() == kEvaluated.
#
#
#
#FailureMessage Object: 0x7ffc01003cf0
----- Native stack trace -----

I split the container into the "build step" using Node v18, and then changed to use the "run step" at Node v20, copying over the node modules, etc., and the above error popped up every time.

So the next best option that I found was to instead use node's slim Debian container instead, so that we could at least get ARM builds out the door. We will have to drop ARMv6 compatibility, but instead we can then add ARM64v8 builds.

I know that Debian's slim containers are larger than Alpine's, but at least we can then have ARMv7 / ARM64v8.

I also had to change the su-exec command in the start-docker.sh to use gosu instead because we're using Debian.

[eliandoran]

LGTM.