Install Package Manager with apt instead of gdebi

[atheriel]

This has a few advantages:

• We don't have to install (and later remove) gdebi.

• apt allows us to skip installing Recommended packages. RSPM depends on git and git recommends less and ssh-client (yes, really), so we were previously installing an SSH client into the image. With this change neither of these packages nor their transitive dependencies are installed.

This shaves about 8M off the resulting image, too.

[glin]

The license manager doesn't seem to work with this change, as the Ubuntu 18 build of it requires libssl1.0.0?

/opt/rstudio-pm/bin/../ext/activation/license-manager: error while loading shared libraries: libcrypto.so.1.0.0: cannot open shared object file: No such file or directory

I didn't realize/forgot that RSPM actually does require libssl via license-manager and other RSW dependencies - rsandbox, launcher. libssl1.0.0 seems to be coming in as a Recommends of git, so that's not great, we should make that an explicit dependency.

And also, the next release of RSPM on Ubuntu 18 uses RSW dependencies built against libssl1.1, so there's some maintenance pain with including libssl manually (e.g. #285).

What do you think of keeping the gdebi change, but not using --no-install-recommends for now? Then we could fix the dependency issues with the RSPM packages for the next release.

[atheriel]

Ah, so really there was an issue with #285 it was just masked by getting pulled in with git. I'd prefer explicitly installing libssl1.0.0 for now, I think, and then fix it once the upstream dependencies are corrected. Reverting back to installing Recommends with a reminder to remove it in the future seems to muddy things even further.

[tylfin]

I think we want to keep the Docker installation as close to the admin guide as possible, because that's the path the RSPM team is testing.

If we want to make a change limiting dependencies, it should happen in the main codebase.

[shepherdjerred]

This would be nice to do. Using gdebi in our official install guide produces a warning that may mislead customers into thinking something is wrong.

https://github.com/rstudio/package-manager/issues/11698

[bschwedler]

We should be able to do this now that Ubuntu 18.04 is EOL.

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