Add opt-in path-style S3 addressing via MCAP_S3_FORCE_PATH_STYLE

[alexhilgart]

Changelog

Add optional path-style S3 addressing support via the MCAP_S3_FORCE_PATH_STYLE environment variable for S3-compatible storage endpoints.

Docs

This PR updates the CLI documentation to include the new MCAP_S3_FORCE_PATH_STYLE environment variable.

Description

Some S3-compatible object storage systems (such as MinIO, Ceph, or custom S3 gateways) require path-style addressing:

https://endpoint/bucket/key

instead of the default virtual-host style:

https://bucket.endpoint/key

The MCAP CLI currently constructs its S3 client using:

s3.NewFromConfig(cfg)

which causes the AWS SDK for Go v2 to use virtual-host style addressing by default. When used with S3-compatible endpoints that do not support bucket subdomains, this can result in DNS errors such as:

lookup <bucket>.<endpoint>: no such host

This PR adds an optional environment variable:

MCAP_S3_FORCE_PATH_STYLE=true

When this variable is set, the S3 client is configured with:

o.UsePathStyle = true

This allows the CLI to work with S3-compatible object stores that require path-style addressing, while preserving the current default behavior for AWS S3 users.

The change is intentionally minimal:

• No behavior changes unless the environment variable is set
• No refactoring of existing reader logic
• Applies to both anonymous and authenticated S3 client initialization paths

Documentation has also been updated in website/docs/guides/cli.md.

Manual testing

Built the CLI locally and verified the behavior against an S3-compatible endpoint.

Without the environment variable, the CLI attempted requests using virtual-host style addressing:

https://bucket.endpoint/key

With the environment variable enabled:

MCAP_S3_FORCE_PATH_STYLE=true

the CLI correctly used path-style addressing:

https://endpoint/bucket/key

and successfully read the remote MCAP file.

Before	After
MCAP CLI always used virtual-host style addressing when accessing S3: https://bucket.endpoint/key This caused DNS failures when the endpoint did not support bucket subdomains.	Path-style addressing can be enabled when needed: MCAP_S3_FORCE_PATH_STYLE=true Resulting requests: https://endpoint/bucket/key This allows the CLI to work with S3-compatible object stores that require path-style addressing.