ci: validate each example's on-chain overlay too - #85

The compose job parses every example's compose.yml and stops there, so not one compose.onchain.yml is checked by anything.

That is the file that most needs it. The offchain path is exercised constantly; the on-chain one needs a wallet, an RPC endpoint and a keystore, so realistically nobody runs it before merging. It is also the more fragile of the two, with a variable per account, path and password.

Compose interpolates ${VAR} as text before parsing the result as YAML, so an unset variable does not error, it becomes an empty string and can silently produce something malformed:

- ${SIGNER_KEYSTORE_DIR}:/keystore:ro     →     - :/keystore:ro
                                                invalid spec: empty section between colons

Interpolating each example's .env.example fixes that, and buys a second check for free: if someone adds a ${VAR} to an overlay and forgets to document it, CI goes red. Without that the failure lands on a user who copied .env.example to .env, filled it in, and still could not start the stack.

--env-file only feeds interpolation; it puts nothing into any container, and .env.example holds placeholders, so no secret is involved. The continue guard keeps an example without an overlay from failing the loop.

Testing

Ran the edited loop verbatim against main on all six examples: api-proxy, echo, hello-world, realtime-transcription, tiles, vllm all pass, and the loop exits 0. So this adds coverage without papering over an existing break.

Same change is going into template-livepeer-runner (#3) and streamdiffusion-livepeer-runner, which have the same gap.