add basic healthcheck support

Dockerfile

@@ -79,3 +79,12 @@ RUN \
     cp ./config/dev.exs ./config/prod.secret.exs && \
     BUILDTIME=1 /entrypoint.sh && \
     rm ./config/prod.secret.exs
+
+# Register healthcheck
+# You might need to change these values on slow or busy servers.
+HEALTHCHECK \
+    --interval=10s \
+    --start-period=50s \
+    --timeout=4s \
+    --retries=3 \
+    CMD curl -sSLf http://localhost:4000/api/pleroma/healthcheck || exit 1

config.dist.exs

@@ -30,6 +30,9 @@ config :pleroma, Pleroma.Repo,
 # You should not change this.
 config :pleroma, Pleroma.Uploaders.Local, uploads: "/uploads"
 
+config :pleroma, :instance,
+  healthcheck: true
+
 #
 # vvv Your awesome config options go here vvv
 #