container fails to start when using readonly filesystem (20-envsubst-on-templates.sh)

### Current Behavior

When mounting the root filesystem with read_only, the entrypoint script `20-envsubst-on-templates.sh` fails to start with the following error:
```
dtrack-frontend-snapshot-1 | /docker-entrypoint.sh: Launching /docker-entrypoint.d/20-envsubst-on-templates.sh
dtrack-frontend-snapshot-1 | 20-envsubst-on-templates.sh: Running envsubst on /etc/nginx/templates/default.conf.template to /etc/nginx/conf.d/default.conf
dtrack-frontend-snapshot-1 | /docker-entrypoint.d/20-envsubst-on-templates.sh: line 53: can't create /etc/nginx/conf.d/default.conf: Read-only file system
```

Tested with image: dependencytrack/frontend:snapshot (image id 1b096cd8f5da)

Readonly root filesystem is working fine using image `dependencytrack/frontend:4.11.4` (image id c3d304f8c999)

Seems like this was introduced with 4ba087690a74f5f1eb302b17142137b233434c2b

### Steps to Reproduce

1. Create a file `compose-snapshot.yml` with the following content:
```
version: "3"

services:
dtrack-frontend:
image: dependencytrack/frontend:snapshot
environment:
- "API_BASE_URL=http://localhost:8081"
ports:
- "8080:8080"
read_only: true
volumes:
- type: tmpfs
target: /tmp
```
2. Start the container:
```
docker compose -f compose-snapshot.yml up

dtrack-frontend-snapshot-1 | /docker-entrypoint.sh: /docker-entrypoint.d/ is not empty, will attempt to perform configuration
dtrack-frontend-snapshot-1 | /docker-entrypoint.sh: Looking for shell scripts in /docker-entrypoint.d/
dtrack-frontend-snapshot-1 | /docker-entrypoint.sh: Launching /docker-entrypoint.d/10-listen-on-ipv6-by-default.sh
dtrack-frontend-snapshot-1 | 10-listen-on-ipv6-by-default.sh: info: can not modify /etc/nginx/conf.d/default.conf (read-only file system?)
dtrack-frontend-snapshot-1 | /docker-entrypoint.sh: Sourcing /docker-entrypoint.d/15-local-resolvers.envsh
dtrack-frontend-snapshot-1 | /docker-entrypoint.sh: Launching /docker-entrypoint.d/20-envsubst-on-templates.sh
dtrack-frontend-snapshot-1 | 20-envsubst-on-templates.sh: Running envsubst on /etc/nginx/templates/default.conf.template to /etc/nginx/conf.d/default.conf
dtrack-frontend-snapshot-1 | /docker-entrypoint.d/20-envsubst-on-templates.sh: line 53: can't create /etc/nginx/conf.d/default.conf: Read-only file system
dtrack-frontend-snapshot-1 exited with code 1
```

### Expected Behavior

I expect root filesystem with readonly to work as before (in 4.11.4):

1. Create a file `compose-4_11_4.yml` with the following content:
```
version: "3"

services:
dtrack-frontend-4-11-4:
image: dependencytrack/frontend:4.11.4
environment:
- "API_BASE_URL=http://localhost:8181"
ports:
- "8180:8180"
read_only: true
volumes:
- type: tmpfs
target: /tmp
```
2. Start the container:
```
docker compose -f compose-4_11_4.yml up

dtrack-frontend-4-11-4-1 | /docker-entrypoint.sh: /docker-entrypoint.d/ is not empty, will attempt to perform configuration
dtrack-frontend-4-11-4-1 | /docker-entrypoint.sh: Looking for shell scripts in /docker-entrypoint.d/
dtrack-frontend-4-11-4-1 | /docker-entrypoint.sh: Launching /docker-entrypoint.d/10-listen-on-ipv6-by-default.sh
dtrack-frontend-4-11-4-1 | 10-listen-on-ipv6-by-default.sh: info: can not modify /etc/nginx/conf.d/default.conf (read-only file system?)
dtrack-frontend-4-11-4-1 | /docker-entrypoint.sh: Sourcing /docker-entrypoint.d/15-local-resolvers.envsh
dtrack-frontend-4-11-4-1 | /docker-entrypoint.sh: Launching /docker-entrypoint.d/20-envsubst-on-templates.sh
dtrack-frontend-4-11-4-1 | /docker-entrypoint.sh: Launching /docker-entrypoint.d/30-oidc-configuration.sh
dtrack-frontend-4-11-4-1 | 30-oidc-configuration.sh: info: can not modify config.json - ENV configuration will be ignored
dtrack-frontend-4-11-4-1 | /docker-entrypoint.sh: Launching /docker-entrypoint.d/30-tune-worker-processes.sh
dtrack-frontend-4-11-4-1 | /docker-entrypoint.sh: Configuration complete; ready for start up
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: using the "epoll" event method
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: nginx/1.25.5
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: built by gcc 13.2.1 20231014 (Alpine 13.2.1_git20231014)
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: OS: Linux 5.15.133.1-microsoft-standard-WSL2
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: getrlimit(RLIMIT_NOFILE): 1024:1048576
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker processes
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 24
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 25
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 26
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 27
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 28
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 29
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 30
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 31
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 32
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 33
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 34
dtrack-frontend-4-11-4-1 | 2024/07/08 09:04:32 [notice] 1#1: start worker process 35
```

### Dependency-Track Frontend Version

4.12.0-SNAPSHOT

### Browser

Mozilla Firefox

### Browser Version

127.0.2

### Operating System

Linux

### Checklist

- [X] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [X] I have checked the [existing issues](https://github.com/DependencyTrack/frontend/issues) for whether this defect was already reported

Start with docker-entrypoint.d/20-envsubst-on-templates.sh, especially the reported line 53, and compare its behavior with the 4.11.4 image. Reproduce using the provided Docker Compose configuration with read_only: true and a /tmp tmpfs; done means the snapshot container starts successfully without the read-only filesystem error.

Written by the indexing model from the issue text.