Replace Traefik with kamal-proxy

[kamal-proxy](https://github.com/basecamp/kamal-proxy) is a custom
minimal proxy designed specifically for Kamal.

It has some 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 a proxy without conflicting config.
3. First class support for Kamal operations - rather than trying to
   work out how to make Traefik do what we want, we can build features
   directly into the proxy, making configuration simpler and avoiding
   obscure errors

test/integration/docker/deployer/app/.kamal/hooks/post-proxy-reboot

@@ -0,0 +1,3 @@
+#!/bin/sh
+echo "Rebooted proxy on ${KAMAL_HOSTS}"
+mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/post-proxy-reboot

test/integration/docker/deployer/app/.kamal/hooks/post-traefik-reboot

@@ -1,3 +0,0 @@
-#!/bin/sh
-echo "Rebooted Traefik on ${KAMAL_HOSTS}"
-mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/post-traefik-reboot

test/integration/docker/deployer/app/.kamal/hooks/pre-proxy-reboot

@@ -0,0 +1,3 @@
+#!/bin/sh
+echo "Rebooting proxy on ${KAMAL_HOSTS}..."
+mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/pre-proxy-reboot

test/integration/docker/deployer/app/.kamal/hooks/pre-traefik-reboot

@@ -1,3 +0,0 @@
-#!/bin/sh
-echo "Rebooting Traefik on ${KAMAL_HOSTS}..."
-mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/pre-traefik-reboot

test/integration/docker/deployer/app/Dockerfile

@@ -6,4 +6,5 @@ ARG COMMIT_SHA
 RUN echo $COMMIT_SHA > /usr/share/nginx/html/version
 RUN mkdir -p /usr/share/nginx/html/versions && echo "version" > /usr/share/nginx/html/versions/$COMMIT_SHA
 RUN mkdir -p /usr/share/nginx/html/versions && echo "hidden" > /usr/share/nginx/html/versions/.hidden
+RUN echo "Up!" > /usr/share/nginx/html/up
 

test/integration/docker/deployer/app/config/deploy.yml

@@ -26,14 +26,11 @@ builder:
   multiarch: false
   args:
     COMMIT_SHA: <%= `git rev-parse HEAD` %>
-healthcheck:
-  cmd: wget -qO- http://localhost > /dev/null || exit 1
-  max_attempts: 3
-traefik:
-  args:
-    accesslog: true
-    accesslog.format: json
-  image: registry:4443/traefik:v2.10
+proxy:
+  image: registry:4443/basecamp/kamal-proxy:latest
+  http_port: 80
+  https_port: 443
+  debug: true
 accessories:
   busybox:
     service: custom-busybox

test/integration/docker/deployer/app_with_roles/.kamal/hooks/post-traefik-reboot

@@ -1,3 +0,0 @@
-#!/bin/sh
-echo "Rebooted Traefik on ${KAMAL_HOSTS}"
-mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/post-traefik-reboot

test/integration/docker/deployer/app_with_roles/.kamal/hooks/pre-traefik-reboot

@@ -1,3 +0,0 @@
-#!/bin/sh
-echo "Rebooting Traefik on ${KAMAL_HOSTS}..."
-mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/pre-traefik-reboot

test/integration/docker/deployer/app_with_roles/Dockerfile

@@ -6,4 +6,4 @@ ARG COMMIT_SHA
 RUN echo $COMMIT_SHA > /usr/share/nginx/html/version
 RUN mkdir -p /usr/share/nginx/html/versions && echo "version" > /usr/share/nginx/html/versions/$COMMIT_SHA
 RUN mkdir -p /usr/share/nginx/html/versions && echo "hidden" > /usr/share/nginx/html/versions/.hidden
-
+RUN echo "Up!" > /usr/share/nginx/html/up

test/integration/docker/deployer/app_with_roles/config/deploy.yml

@@ -20,14 +20,8 @@ builder:
   multiarch: false
   args:
     COMMIT_SHA: <%= `git rev-parse HEAD` %>
-healthcheck:
-  cmd: wget -qO- http://localhost > /dev/null || exit 1
-  max_attempts: 3
-traefik:
-  args:
-    accesslog: true
-    accesslog.format: json
-  image: registry:4443/traefik:v2.10
+proxy:
+  image: registry:4443/basecamp/kamal-proxy:latest
 accessories:
   busybox:
     service: custom-busybox

test/integration/docker/deployer/setup.sh

@@ -19,7 +19,7 @@ push_image_to_registry_4443() {
 
 install_kamal
 push_image_to_registry_4443 nginx 1-alpine-slim
-push_image_to_registry_4443 traefik v2.10
+push_image_to_registry_4443 basecamp/kamal-proxy latest
 push_image_to_registry_4443 busybox 1.36.0
 
 # .ssh is on a shared volume that persists between runs. Clean it up as the