feat(render): specify Kroki instance through KROKI_ENDPOINT environ…

…ment variable

src/Kroki.jl

@@ -117,11 +117,16 @@ RenderError(::Diagram, exception::Exception) = exception
 """
 Renders a [`Diagram`](@ref) through a Kroki service to the specified output
 format.
+
+A `KROKI_ENDPOINT` environment variable can be set, specifying the URI of a
+specific instance of Kroki to use (e.g. when using a [privately hosted
+instance](https://docs.kroki.io/kroki/setup/install/)). By default the
+[publicly hosted service](https://kroki.io) is used.
 """
 render(diagram::Diagram{T}, output_format::AbstractString) where T <: Val = try
   getfield(
     request("GET", join([
-      "https://kroki.io",
+      get(ENV, "KROKI_ENDPOINT", "https://kroki.io"),
       lowercase("$(T.parameters[1])"),
       output_format,
       UriSafeBase64Payload(diagram)

test/runtests.jl

@@ -108,6 +108,30 @@ end
         InvalidOutputFormatError
       )
     end
+
+    @testset "`KROKI_ENDPOINT` environment variable" begin
+      # The instance of the Kroki service that is used for rendering can be
+      # controlled through the `KROKI_ENDPOINT` environment variable. The most
+      # straight-forward way for testing this is by pointing to an invalid
+      # endpoint and testing for a corresponding connection error
+      expected_diagram_type = :plantuml
+      expected_service_host = "https://localhost"
+
+      withenv("KROKI_ENDPOINT" => expected_service_host) do
+        try
+          render(Diagram(expected_diagram_type, "A -> B: C"), "svg")
+        catch exception
+          buffer = IOBuffer()
+          showerror(buffer, exception)
+          rendered_buffer = String(take!(buffer))
+
+          @test occursin("ECONNREFUSED", rendered_buffer)
+          @test occursin(
+            "$(expected_service_host)/$(expected_diagram_type)", rendered_buffer
+          )
+        end
+      end
+    end
   end
 end