Add example for GitHub Actions + Proxy

### Question

I can't get my Github Actions runner to properly connect to my MySQL Cloud SQL instance using the cloud-sql-proxy Docker container.

These are the relevant steps of my GHA YML:

```yml
steps:
- name: Set up Go
uses: actions/setup-go@v4
with:
go-version: '1.21'

- name: Checkout
uses: actions/checkout@v3

# Setup gcloud CLI
- uses: google-github-actions/setup-gcloud@v0
with:
service_account_key: ${{ secrets.AUTH_KEY }}
project_id: ${{ env.PROJECT_ID }}
export_default_credentials: true

- name: Run database migrations
run: |
chmod -R +x .
make proxy_db
make migration
```

I can see in the action runner logs that `${{ secrets.AUTH_KEY }}` and `${{ env.PROJECT_ID }}` are being parsed correctly.

These are the definitions of `make proxy_db` and `make migration` in my makefile:

```lang-makefile
proxy_db:
docker run -d \
-p 127.0.0.1:3306:3306 \
-v $(GOOGLE_APPLICATION_CREDENTIALS):/config \
gcr.io/cloudsql-docker/gce-proxy:1.16 /cloud_sql_proxy \
-instances=$(CLOUD_SQL_INSTANCE_NAME)=tcp:0.0.0.0:3306 -credential_file=/config
```
```lang-makefile
migration:
@go run cmd/migration/main.go
```

The contents of `main.go` are as follows:
```go
package main

import (
"database/sql"
"fmt"
"time"

_ "github.com/GoogleCloudPlatform/berglas/pkg/auto"
"github.com/golang-migrate/migrate/v4"
"github.com/golang-migrate/migrate/v4/database/mysql"
_ "github.com/golang-migrate/migrate/v4/database/mysql"
_ "github.com/golang-migrate/migrate/v4/source/file"
"github.com/kelseyhightower/envconfig"
)

type variables struct {
MySQLProxyPort int `required:"false" envconfig:"mysql_proxy_port"`
MySQLDB string `required:"true" envconfig:"mysql_db"`
MySQLUser string `required:"true" envconfig:"mysql_user"`
MySQLPass string `required:"true" envconfig:"mysql_pass"`
Env string `envconfig:"build_env"`
}

const migrationsPath = "scripts/db/migrations"

func main() {
var v variables
envconfig.MustProcess("db migration", &v)

db, err := sql.Open("mysql", getDbSource(v))
db.SetConnMaxLifetime(time.Minute * 5)
source := fmt.Sprintf("file://%s", migrationsPath)
driver, err := mysql.WithInstance(db, &mysql.Config{DatabaseName: v.MySQLDB, StatementTimeout: 300 * time.Second})
if err != nil {
panic(err)
}
m, err := migrate.NewWithDatabaseInstance(source, v.MySQLDB, driver)
if err != nil {
panic(err)
}

fmt.Println("Running migrations...")
if err := m.Up(); err != nil {
if err == migrate.ErrNoChange {
fmt.Println("No migrations to run.")
return
}

panic(err)
}
fmt.Println("Migrations ran successfully.")
}

func getDbSource(v variables) string {
tls := fmt.Sprintf("?tls=%t", v.Env == "live")
return fmt.Sprintf(
"%s:%s@tcp(localhost:%v)/%s%s",
v.MySQLUser,
v.MySQLPass,
v.MySQLProxyPort,
v.MySQLDB,
tls,
)
}

func getDbUrl(v variables) string {
return fmt.Sprintf(
"mysql://%s", getDbSource(v),
)
}

```

From action runner logs, I can see that my environment variables (including those which are references to Google Secret Manager secrets and are processed by envconfig) are being processed correctly, so I don't believe there is a problem with my `GOOGLE_APPLICATION_CREDENTIALS`. My `migrationsPath` variable in main.go pointing to where my migrations live also doesn't contain any typos.

When this action runs in Github Actions, it downloads the container, outputs a digest message, outputs a status message, and then outputs a hash. It does not output any error messages.

It then tries to run migrations. After about a minute, the Github Action exits with these error logs:

```
panic: dial tcp 127.0.0.1:3306: connect: connection refused

goroutine 1 [running]:
main.main()
/home/runner/.../cmd/migration/main.go:35 +0x465
exit status 2
make: *** [makefile:14: migration] Error 1
Error: Process completed with exit code 2.
```

I've also tried this with a `wait` script between the two commands of my GHA step, but that didn't make a difference, it says the DB is ready after 0 seconds. I think that the container is being created, but that is not running.

I've also tried using a v1 container:
```bash
docker run -d \
-p 127.0.0.1:3306:3306 \
-v $(GOOGLE_APPLICATION_CREDENTIALS):/config \
gcr.io/cloudsql-docker/gce-proxy:1.16 /cloud_sql_proxy \
-instances=$(CLOUD_SQL_INSTANCE_NAME)=tcp:0.0.0.0:3306 -credential_file=/config
```
but this doesn't work either, same error.

Port 3306 is open in my project's firewall settings.

What do I need to do to get it to work? On previous projects I've collaborated on, this approach works without issue for Postgres databases, but I can't get it to actually work on my own GCS project.

### Code

_No response_

### Additional Details

_No response_