Docker + Golang HTTPS issue. OpenSSL SSL_connect: SSL_ERROR_SYSCALL

I'm trying to connect to an HTTPS session created in Go from a Docker container, it works fine when I run it on my local, but as soon as I try to run it in the container I can't access to the URL.

func main() {

	// I'm using Go-Chi V5
	r := chi.NewRouter()

	// TLS connection. Inspired from https://blog.cloudflare.com/exposing-go-on-the-internet
	tlsConfig := &tls.Config{
		PreferServerCipherSuites: true,
		CurvePreferences: []tls.CurveID{
			tls.CurveP256,
			tls.X25519,
		},

		MinVersion: tls.VersionTLS12,
		CipherSuites: []uint16{
			tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
			tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
			tls.TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305,
			tls.TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305,
			tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
			tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
		},
	}
	srv := &http.Server{
		Addr:         "localhost:9090",
		ReadTimeout:  5 * time.Second,
		WriteTimeout: 20 * time.Second,
		IdleTimeout:  200 * time.Second,
		TLSConfig:    tlsConfig,
		Handler:      r,
	}

	r.Get("/", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("hello world"))
	})

	err := srv.ListenAndServeTLS("./.dev/localhost.crt", "./.dev/localhost.key")
	if err != nil {
		log.Fatalf("server failed to start: %v", err)
	}
}

My Dockerfile:

FROM golang:1.17-buster AS build

WORKDIR /app

COPY go.mod .
COPY go.sum .
RUN go mod download

COPY ./.dev/localhost.key .
COPY ./.dev/localhost.crt .

COPY . .

RUN go build -o /dev-admin ./cmd/admin

##
## Deploy
##

FROM gcr.io/distroless/base-debian10

WORKDIR /

COPY --from=build /dev-admin /dev-admin

COPY --from=build ./app/localhost.key ./.dev/localhost.key
COPY --from=build ./app/localhost.crt ./.dev/localhost.crt

EXPOSE 9090
EXPOSE 443

ENTRYPOINT ["/dev-admin"]

docker-compose.yml:

version: '3.9'
services:
  web:
    image: api_test
    build: .
    ports:
      - '9090:9090'
    volumes:
      - .:/app

$ curl https://localhost:9090
curl: (35) OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to localhost:9090

The container works fine when I try the normal HTTP version, so there is a blockage on the HTTPS only and I can't figure out what it is.

You are specifying the address to listen on in your Server definition:

srv := &http.Server{
        Addr:         "localhost:9090",
...

change this to:

srv := &http.Server{
        Addr:         ":9090",
...

The reason that this change is needed is that the address "specifies the TCP address for the server to listen on". When you specify localhost:9090 you are binding to the loopback interface within the container and, in most cases, this is not accessible externally (see this question for more info).