[Docs] Add installation option for inside Docker/Containers

[rrmistry]

What problem are you trying to solve?

Attempting to install devbox inside docker using the provided Linux script doesn't work because of the prompt messages that get asked.

E.g.

curl -fsSL https://get.jetify.com/devbox | bash
...
? Install devbox to /usr/local/bin (requires sudo)? [Y/n]
...
Nix is not installed. Devbox will attempt to install it.

Press enter to continue or ctrl-c to exit.
...

From Discord, I found this script:

# Step 2: Installing Nix
RUN wget --output-document=/dev/stdout https://nixos.org/nix/install | sh -s -- --no-daemon
RUN . ~/.nix-profile/etc/profile.d/nix.sh

ENV PATH="/home/${DEVBOX_USER}/.nix-profile/bin:$PATH"

# Step 3: Installing devbox
ENV DEVBOX_USE_VERSION=$DEVBOX_USE_VERSION
RUN wget --quiet --output-document=/dev/stdout https://get.jetify.com/devbox   | bash -s -- -f
RUN chown -R "${DEVBOX_USER}:${DEVBOX_USER}" /usr/local/bin/devbox

But this was not easy to find.

What solution would you like?

Documenting this front-and-centre in the official docs would be good.