Language provider from `nixpacks.toml` is not used

Question

Project: `f6bf90d2-8b23-4b9c-bfb2-49eabce8e261`

Contents of `nixpacks.toml`:
```
providers = ["java"]
```

Contents of `railway.toml`:
```
[build.nixpacksPlan.phases.install]
cmds = [ "mkdir -p ./shared-jvmhack/src/main && cp -R ./shared/src/commonMain/kotlin ./shared-jvmhack/src/main/kotlin" ]

[build]
builder = "NIXPACKS"
buildCommand = "./gradlew clean application:build -x check"

[deploy]
startCommand = "java -XX:+UseZGC -XX:SoftMaxHeapSize=200m -Xms30m -Xmx1500m -XX:+PrintCommandLineFlags --show-version -jar ./application/build/quarkus-app/quarkus-run.jar"
```

No provider gets used:
```
 
[Region: europe-west4]
==============
Using Nixpacks
==============
context: eee02ec4c708c0708ea8ebb7e0cbc134
 
╔══════════════════════════════ Nixpacks v1.26.0 ══════════════════════════════╗
║ install    │ mkdir -p ./shared-jvmhack/src/main && cp -R                     ║
║            │ ./shared/src/commonMain/kotlin ./shared-jvmhack/src/main/kotlin ║
║──────────────────────────────────────────────────────────────────────────────║
║ build      │ ./gradlew clean application:build -x check                      ║
║──────────────────────────────────────────────────────────────────────────────║
║ start      │ java -XX:+UseZGC -XX:SoftMaxHeapSize=200m -Xms30m -Xmx1500m     ║
║            │ -XX:+PrintCommandLineFlags --show-version -jar                  ║
║            │ ./application/build/quarkus-app/quarkus-run.jar                 ║
╚══════════════════════════════════════════════════════════════════════════════╝

#0 building with "builder-51zY" instance using docker-container driver
 
#1 [internal] load build definition from Dockerfile
...
BUILD FAILED
```
-------------

The result is identical if I specify the provider as `providers = ["...", "java"]` to allow for provider autodiscovery.