[mproxy](https://github.com/kevinmcconnell/mproxy) is a custom minimal
proxy designed specifically for Kamal.
It has two big advantages over Traefik:
1. Imperative deployments - we tell it to switch from container A to
container B, and it waits for container B to start then switches. No
need to poll for health checks ourselves or mess around with forcing
health checks to fail.
2. Support for multiple apps - as much as possible, configuration is
supplied at runtime by the deploy command, allowing us to have
multiple apps share an instance of mproxy without conflicting config.
This will allow us to filter for containers that have no destination in
cases where we deploy an empty + a non empty destination to the same
host.
To note:
```
\# Containers with a destination label
$ docker ps --filter label=destination
\# Containers with an empty destination label
$ docker ps --filter label=destination=
```
If no context is specified and we are in a git repo, then we'll build
from a git archive by default. This means we don't need a separate
setting and gives us a safer default build.
Building directly from a checkout will pull in uncommitted files to or
more sneakily files that are git ignored, but not docker ignored.
To avoid this, we'll add an option to build from a git archive of HEAD
instead. Docker doesn't provide a way to build directly from a git
repo, so instead we create a tarball of the current HEAD with git
archive and pipe it into the build command.
When building from a git archive, we'll still display the warning about
uncommitted changes, but we won't add the `_uncommitted_...` suffix to
the container name as they won't be included in the build.
Perhaps this should be the default, but we'll leave that decision for
now.
Secret and clear env variables have different lifecycles. The clear ones
are part of the repo, so it makes sense to always deploy them with the
rest of the repo.
The secret ones are external so we can't be sure that they are up to
date, therefore they require an explicit push via `envify` or `env push`.
We'll keep the env file, but now it just contains secrets. The clear
values are passed directly to `docker run`.
Add an app with roles to the integration tests. We'll deploy two web
containers and one worker. The worker just sleeps, so we are testing
that the container has booted.
If curl is not available to download the docker install script, try
with wget instead.
If neither is available or both fail, return a simple failing script
so that we don't carry on regardless.
Fixes: https://github.com/basecamp/kamal/issues/395
This allows the user to make any necessary configuration changes to
Docker before setting up any containers, allowing those configuration
changes to take effect from the outset.
