Fix: docker-compose builder supports v2.x releases (exec format error)

[64johnlee]

Summary

Fixes exec format error when using Docker Compose v2.x with the community builder (issue #616).

Root Cause

Docker Compose v2.x changed the release binary naming convention on GitHub:

Version	Binary name
v1.x	docker-compose-Linux-x86_64 (OS capitalized)
v2.x	docker-compose-linux-x86_64 (OS lowercase)

The Dockerfile used $(uname -s) which returns Linux (capital L). For v2.x releases this produces a 404 URL. Because curl was called without --fail, it silently wrote the HTTP error page to disk. After chmod +x succeeded, trying to execute the HTML file produces:

exec /usr/local/bin/docker-compose: exec format error

Fix

Dockerfile:

• Pipe uname -s through tr '[:upper:]' '[:lower:]' to produce the lowercase OS name needed by v2.x
• Add --fail to curl so HTTP errors (404) abort the build immediately instead of producing a silently broken binary
• Update base image from ubuntu:bionic (EOL April 2023) to ubuntu:focal (LTS, supported until April 2025)

cloudbuild.yaml:

• Update default _DOCKER_COMPOSE_VERSION from 1.28.2 to 2.29.7 (current stable v2 release)

Backward Compatibility

• Users passing an explicit --substitutions _DOCKER_COMPOSE_VERSION=X.Y.Z will also need to use v2.x format versions
• v1.x is EOL (end of life since July 2023); migration to v2 is the official path
• The v prefix added to the release tag URL works for all Docker Compose releases (both v1.x and v2.x tags on GitHub include the v prefix)

Example

# Build with Docker Compose v2.29.7 (default)
steps:
- name: 'gcr.io/$PROJECT_ID/docker-compose'
  args: ['up', '-d']

# Or specify a version explicitly
# gcloud builds submit . --substitutions _DOCKER_COMPOSE_VERSION=2.25.0

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