As it turned out, things getting really confusing here. 1. Downgrading mesa to 18.6.2 as suggested only works for an ARMv7 Installation, so using a different base image. However, after downgrading, the browser starts to work but glxinfo | grep render still yields:
$this->bbcode_second_pass_code('', '
direct rendering: Yes
GLX_MESA_multithread_makecurrent, GLX_MESA_query_renderer,
GLX_EXT_visual_rating, GLX_MESA_copy_sub_buffer, GLX_MESA_query_renderer,
Extended renderer info (GLX_MESA_query_renderer):
OpenGL renderer string: VC4 V3D 2.1
GL_OES_fbo_render_mipmap, GL_OES_get_program_binary, GL_OES_mapbuffer,
')
so no Gallium3D anywhere.
However that does not change a thing for an AArch64 Installation downgrading mesa here has no effect at all.
I would really be curious to know why that is and if there is any way to get that up and running on AArch64 as well.