feat: add SUPABASE_HOSTNAME env var to override local service host

[kanaka]

Allows the CLI to work correctly when run inside a dev container with the Docker socket bind-mounted. In that context 127.0.0.1 is the container's own loopback, not the Docker host.

Set SUPABASE_HOSTNAME=host.docker.internal to reach sibling containers via Docker Desktop's host gateway without proxying the Docker socket.

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AI assistance: Claude Sonnet 4.6 in Pi Agent

• Used to find root cause, propose fix options, draft code/test/docs.
• I ran supabase start inside the dev container, confirmed all services healthy, reviewed/edited all code changes/text.

What kind of change does this PR introduce?

Feature

What is the current behavior?

If supbase start is run from a sibling container (e.g. with /var/run/docker.sock mounted from the host), the command is unable to connect to the those containers after starting them in order to do health checks:

$ cli/supabase start
supabase_rest_mal container logs:
10/Mar/2026:16:01:56 +0000: Starting PostgREST 14.5...
10/Mar/2026:16:01:56 +0000: Admin server listening on 0.0.0.0:3001
...
Stopping containers...
failed to execute http request: Head "http://127.0.0.1:54321/rest-admin/v1/ready": dial tcp 127.0.0.1:54321: connect: connection refused
failed to execute http request: Head "http://127.0.0.1:54321/functions/v1/_internal/health": dial tcp 127.0.0.1:54321: connect: connection refused

What is the new behavior?

$ SUPABASE_HOSTNAME=host.docker.internal cli/supabase start
Starting containers...
Started supabase local development setup.
...

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Central YAML (base), Organization UI (inherited)

Review profile: CHILL

Plan: Pro

Run ID: 880e4b38-efd0-4e22-8cc7-1f1e0b43ccd8

📥 Commits

Reviewing files that changed from the base of the PR and between 0335b24 and bbceb86.

📒 Files selected for processing (3)

• docs/supabase/start.md
• internal/utils/misc.go
• internal/utils/misc_test.go

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • CLI now supports environment variable configuration for hostname resolution in containerized environments.

• Documentation

  • Added guidance for configuring the CLI when running inside Docker containers with socket bind-mounting.

• Tests

  • Added test coverage for hostname environment variable configuration.

Walkthrough

The changes add support for a new SUPABASE_HOSTNAME environment variable to allow users to override the hostname used when the CLI runs inside a dev container with Docker socket bind-mounted. The GetHostname function in internal/utils/misc.go is updated to check this environment variable first, and if set, returns its value; otherwise, it retains the existing logic of deriving the host from Docker.DaemonHost(). Documentation is added to advise users on setting this variable, and tests are added to verify the environment-based behavior of the GetHostname function.

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[coveralls]

Pull Request Test Coverage Report for Build 22911967653

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.005%) to 61.731%

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Files with Coverage Reduction	New Missed Lines	%
internal/utils/git.go	5	57.14%

Totals
Change from base Build 22906505523:	-0.005%
Covered Lines:	7725
Relevant Lines:	12514

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💛 - Coveralls