Allow hostname without github-runner prefix to avoid 64-char limit

[ElsevierAlex]

Fix: Allow runner name to use hostname without the github-runner- prefix to avoid exceeding GitHub's 64-character limit for runner names.

Problem Description

Currently, the runner name generation logic in [entrypoint.sh](https://github.com/myoung34/docker-github-actions-runner/blob/master/entrypoint.sh#L45) always includes a prefix when using the hostname, which can cause the total runner name to exceed GitHub's 64-character limit.
The hostname length can be controlled by the consumer to be up to 64 characters, but the prefix may cause the limit to be exceeded, resulting in the runner failing to be created with the following error:
An error occurred: Not configured. Run config.(sh/cmd) to configure the runner.

This is problematic in cases where:

• Adding the default prefix github-runner- (14 characters) + hostname would exceed the 64-character limit
• The current logic forces a dash separator even when RUNNER_NAME_PREFIX is empty

[myoung34]

This could be a breaking change.

Before: it would be github-runner-{hostname} with nothing set.
While your changes work, the after is now hostname with nothing set

If you Default RUNNER_NAME_PREFIX to github-runner I'll approve this as it means no one will need to change anything, and you can use RUNNER_NAME_PREFIX="" to get the new behavior for yourself

[ElsevierAlex]

This could be a breaking change.

Before: it would be github-runner-{hostname} with nothing set. While your changes work, the after is now hostname with nothing set

If you Default RUNNER_NAME_PREFIX to github-runner I'll approve this as it means no one will need to change anything, and you can use RUNNER_NAME_PREFIX="" to get the new behavior for yourself

This could be a breaking change.

Before: it would be github-runner-{hostname} with nothing set. While your changes work, the after is now hostname with nothing set

If you Default RUNNER_NAME_PREFIX to github-runner I'll approve this as it means no one will need to change anything, and you can use RUNNER_NAME_PREFIX="" to get the new behavior for yourself

Addressed