Added an integration test for proxied accessory using Busybox and netcat

test/integration/docker/deployer/app_with_proxied_accessory/Dockerfile

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+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_proxied_accessory/config/deploy.yml

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+service: app_with_proxied_accessory
+image: app_with_proxied_accessory
+servers:
+  - vm1
+env:
+  clear:
+    CLEAR_TOKEN: 4321
+    CLEAR_TAG: ""
+    HOST_TOKEN: "${HOST_TOKEN}"
+asset_path: /usr/share/nginx/html/versions
+proxy:
+  host: 127.0.0.1
+registry:
+  server: registry:4443
+  username: root
+  password: root
+builder:
+  driver: docker
+  arch: <%= Kamal::Utils.docker_arch %>
+  args:
+    COMMIT_SHA: <%= `git rev-parse HEAD` %>
+accessories:
+  busybox:
+    service: custom-busybox
+    image: registry:4443/busybox:1.36.0
+    cmd: sh -c 'echo "Starting busybox..."; trap exit term; while true; do sleep 1; done'
+    roles:
+      - web
+  netcat:
+    service: netcat
+    image: registry:4443/busybox:1.36.0
+    cmd: >
+      sh -c 'echo "Starting netcat..."; while true; do echo -e "HTTP/1.1 200 OK\r\nContent-Length: 11\r\n\r\nHello Ruby" | nc -l -p 80; done'
+    roles:
+      - web
+    port: 12345:80
+    proxy:
+      host: netcat
+      ssl: false
+      healthcheck:
+        interval: 1
+        timeout: 1
+        path: "/"
+

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

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+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;
+    }
+}