Move installation of grav in Dockerfile

[fabrizio2210]

I think it is better to move the installation of the grav in Dockerfile. This move give us:

• faster start of the container
• possibility to use volume in path /usr/html/user. If the file are already exist during the execution a volume can be mounted, else the run.sh blocks during the "mv" command

[dominictayloruk]

This merge is no good, sorry.

1, Moving installation to Dockerfile is also no good for this image as you can clone from a git repo upon container start rather than the default Grav repo & as to not have to worry too much about persistent storage + easier to scale multiple pods (f5ef428)

2, No need for cross building as they are all built on native architecture to reduce these unnecessary layers (b8c0368)

3, :latest already contains admin, no need to transform from deliver. :deliver-site contains deliver and there is also :one-page which contains one page (45e954b)