Before this patch, a missing scheme would sometimes produce a confusing
error message. If no scheme was specified at all, an empty "" would be
included in the message;

    echo '{"registry-mirrors":["example.com"]}' > my-config.json
    dockerd --config-file ./my-config.json
    # ...
    failed to start daemon: invalid mirror: unsupported scheme "" in "example.com"

If a scheme was missing, but a port was included, the hostname would be
printed as the scheme;

    echo '{"registry-mirrors":["example.com:8080"]}' > my-config.json
    dockerd --config-file ./my-config.json
    # ...
    failed to start daemon: invalid mirror: unsupported scheme "example.com" in "example.com:8080"

With this patch applied, the error messages are slightly more user-friendly;

    echo '{"registry-mirrors":["example.com"]}' > my-config.json
    dockerd --config-file ./my-config.json
    # ...
    failed to start daemon: invalid mirror: no scheme specified for "example.com": must use either 'https://' or 'http://'

    echo '{"registry-mirrors":["example.com:8080"]}' > my-config.json
    dockerd --config-file ./my-config.json
    # ...
    failed to start daemon: invalid mirror: no scheme specified for "example.com:8080": must use either 'https://' or 'http://'

Signed-off-by: Sebastiaan van Stijn <[email protected]>
(cherry picked from commit 307c185)
Signed-off-by: Sebastiaan van Stijn <[email protected]>

Validation of registry mirrors was performed during daemon startup,
but after the config-file was validated. As a result, the `--validate`
option would incorrectly print that the configuration was valid, but
the daemon would fail to start;

    echo '{"registry-mirrors":["example.com"]}' > my-config.json
    dockerd --config-file ./my-config.json --validate
    configuration OK

    dockerd --config-file ./my-config.json
    # ...
    failed to start daemon: invalid mirror: no scheme specified for "example.com": must use either 'https://' or 'http://'

With this patch applied, validation is also performed as part of the
daemon config validation;

    echo '{"registry-mirrors":["example.com"]}' > my-config.json
    dockerd --config-file ./my-config.json --validate
    unable to configure the Docker daemon with file ./my-config.json: merged configuration validation from file and command line flags failed: invalid mirror: no scheme specified for "example.com": must use either 'https://' or 'http://'

    # fix the invalid config
    echo '{"registry-mirrors":["https://example.com"]}' > my-config.json
    dockerd --config-file ./my-config.json --validate
    configuration OK

Signed-off-by: Sebastiaan van Stijn <[email protected]>
(cherry picked from commit 1d8545d)
Signed-off-by: Sebastiaan van Stijn <[email protected]>