enhancement(docker platform): Dockerfile updates and improvements (#13483)

* enhancement(docker platform): enhancements to dockerfiles to better follow spec

Signed-off-by: Spencer Gilbert <spencer.gilbert@datadoghq.com>

* docker-entrypoint fixings

Signed-off-by: Spencer Gilbert <spencer.gilbert@datadoghq.com>

* Update distribution/docker/alpine/Dockerfile

Co-authored-by: Hugo Hromic <hhromic@users.noreply.github.com>

* fix chown and match alpine & debian user/group commands

Signed-off-by: Spencer Gilbert <spencer.gilbert@datadoghq.com>

* remove gosu, document docker-entrypoint

Signed-off-by: Spencer Gilbert <spencer.gilbert@datadoghq.com>
Co-authored-by: Hugo Hromic <hhromic@users.noreply.github.com>