Proxy boot config

Add commands for managing proxy boot config. Since the proxy can be
shared by multiple applications, the configuration doesn't belong in
`config/deploy.yml`.

Instead you can set the config with:

```
Usage:
  kamal proxy boot_config <set|get|clear>

Options:
      [--publish], [--no-publish], [--skip-publish]   # Publish the proxy ports on the host
                                                      # Default: true
      [--http-port=N]                                 # HTTP port to publish on the host
                                                      # Default: 80
      [--https-port=N]                                # HTTPS port to publish on the host
                                                      # Default: 443
      [--docker-options=option=value option2=value2]  # Docker options to pass to the proxy container
```

By default we boot the proxy with `--publish 80:80 --publish 443:443`.

You can stop it from publishing ports, specify different ports and pass
other docker options.

The config is stored in `.kamal/proxy/options` as arguments to be passed
verbatim to docker run.

Where someone wants to set the options in their application they can do
that by calling `kamal proxy boot_config set` in a pre-deploy hook.

There's an example in the integration tests showing how to use this to
front kamal-proxy with Traefik, using an accessory.

test/integration/docker/deployer/Dockerfile

@@ -19,6 +19,7 @@ RUN apt-get update --fix-missing && apt-get install -y docker-ce docker-ce-cli c
 COPY *.sh .
 COPY app/ app/
 COPY app_with_roles/ app_with_roles/
+COPY app_with_traefik/ app_with_traefik/
 
 RUN rm -rf /root/.ssh
 RUN ln -s /shared/ssh /root/.ssh
@@ -28,6 +29,7 @@ RUN git config --global user.email "deployer@example.com"
 RUN git config --global user.name "Deployer"
 RUN cd app && git init && git add . && git commit -am "Initial version"
 RUN cd app_with_roles && git init && git add . && git commit -am "Initial version"
+RUN cd app_with_traefik && git init && git add . && git commit -am "Initial version"
 
 HEALTHCHECK --interval=1s CMD pgrep sleep
 

test/integration/docker/deployer/app_with_traefik/.kamal/hooks/pre-deploy

@@ -0,0 +1,3 @@
+kamal proxy boot_config set --publish false \
+                            --docker_options label=traefik.http.services.kamal_proxy.loadbalancer.server.scheme=http \
+                                             label=traefik.http.routers.kamal_proxy.rule=PathPrefix\(\`/\`\)

test/integration/docker/deployer/app_with_traefik/.kamal/secrets

@@ -0,0 +1 @@
+SECRET_TOKEN='1234 with "中文"'

test/integration/docker/deployer/app_with_traefik/Dockerfile

@@ -0,0 +1,9 @@
+FROM registry:4443/nginx:1-alpine-slim
+
+COPY default.conf /etc/nginx/conf.d/default.conf
+
+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_traefik/config/deploy.yml

@@ -0,0 +1,29 @@
+service: app_with_traefik
+image: app_with_traefik
+servers:
+  - vm1
+  - vm2
+deploy_timeout: 2
+drain_timeout: 2
+readiness_delay: 0
+
+registry:
+  server: registry:4443
+  username: root
+  password: root
+builder:
+  driver: docker
+  arch: <%= Kamal::Utils.docker_arch %>
+  args:
+    COMMIT_SHA: <%= `git rev-parse HEAD` %>
+accessories:
+  traefik:
+    service: traefik
+    image: traefik:v2.10
+    port: 80
+    cmd: "--providers.docker"
+    options:
+      volume:
+        - "/var/run/docker.sock:/var/run/docker.sock"
+    roles:
+      - web

test/integration/docker/deployer/app_with_traefik/default.conf

@@ -0,0 +1,17 @@
+server {
+    listen       80;
+    listen  [::]:80;
+    server_name  localhost;
+
+    location / {
+        root   /usr/share/nginx/html;
+        index  index.html index.htm;
+    }
+
+    # redirect server error pages to the static page /50x.html
+    #
+    error_page   500 502 503 504  /50x.html;
+    location = /50x.html {
+        root   /usr/share/nginx/html;
+    }
+}