bug: Add libcurl4 to Dockerfile

[jrconlin]

Closes #94

Description

App was failing to launch because libcurl4 was not being loaded into the app_1 dockerfile.

Testing

ROCKET_BROADCASTER_AUTH={"test"=["foobar"]} ROCKET_READER_AUTH={"autopush"=["quux"]} docker-compose up

should report app_1 starting. You can spot this in the log file, via something similar to:

app_1  | {"Logger":"megaphone-0.1.6","Type":"megaphone:log","Hostname":"...","Pid":1,"Severity":6,"Timestamp":...,"Fields":{"msg":"Starting up"}}

Issue(s)

Closes #94.

[pjenvey]

I'm a bit concerned this could randomly break the build in the future. It's been a while since it's happened, but I recall Rocket occasionally having issues with the latest nightly

[jrconlin]

I suppose, I mostly updated to normal nightly because it's far easier to forget to update this with the "latest" version. If we hit a problem again, we can freeze until we know it works again.