install script should check to see if 'curl' is installed

[mrphs]

'curl' isn't installed by default on Debian. The installation script tries to run docker installation script by fetching it via 'curl' but if it isn't installed, it fails all together.

The safest bet would be to make sure 'curl' is installed before getting to other steps.

[trevj]

@mrphs Thanks for trying Outline. Seems like we could add instructions for both wget and curl.

[staticfrost]

I'll take a look at this one. https://github.com/Jigsaw-Code/outline-server/pull/345

[hamm3l]

Would like to revive this as this is still an issue particularly on Debian 12.2 netinst, which does not come with curl by default (as is the case on many minimal Linux distro installs). It would be easy to add a check to the install bash script, and then prompt the user to install curl through their package manager. Right now the script simply fails stating it couldn't install docker, which is not helpful as one has to then go through the install script to find that the script calls curl.