Support digest for input image in convertor

[tianouya-db]

What this PR does / why we need it:

The userspace convertor requires an --input-tag to identify the image to convert (USERSPACE_CONVERTOR.md).

-i, --input-tag string          tag for image converting from (required)

This is sometimes inconvenient because:

• A tag can be volatile
• Sometimes the client has access to the digest but not the tags on the image to convert

This PR supports a parameter to specify the digest of the image to convert, as an enhancement.

-g, input-digest             digest for image converting from (required when input-tag is not set)

With this change, the command accepts either a tag (--input-tag) or a digest (--input-digest) to specify the image to convert.

Example usage with the digest parameter:

bin/convertor -r docker.io/overlaybd/redis -u user:pass -g sha256:309f99718ff2424f4ae5ebf0e46f7f0ce03058bf47d9061d1d66e4af53b70ffc -o 309f99718ff2424f4ae5ebf0e46f7f0ce03058bf47d9061d1d66e4af53b70ffc_obd

Manual test

I built the convertor locally, and ran it to convert an image with digest to verify it works:

sudo bin/convertor --mkfs --repository my.registry.com/repo --username <user>:<password>--input-digest sha256:ea1b8ca870974180c0f369eda3dbb529e06303ff8b519a237573002d7ed85bee --overlaybd 0.1-overlaybd

INFO[0000] building [Overlaybd - Native]  image...
INFO[0000] building tmp_conv/1--ea1b8ca870974180c0f369eda3dbb529e06303ff8b519a237573002d7ed85bee ...
INFO[0000] fetching manifest sha256:ea1b8ca870974180c0f369eda3dbb529e06303ff8b519a237573002d7ed85bee with type application/vnd.docker.distribution.manifest.v2+json
INFO[0002] downloaded layer 0
INFO[0002] mkfs for baselayer, vsize: 64 GB
INFO[0003] layer 0 committed, uuid: bce45ce6-13d3-4bff-6a34-04a4c2d56a5f, parent uuid:
INFO[0003] layer 0 converted
INFO[0007] layer 0 uploaded
INFO[0008] config uploaded
INFO[0009] manifest uploaded, sha256:2446e61a1ea59a980b96e6e1e32b639ac4be4a05b64cb45ee2c8b4bd7f9c038e
INFO[0009] convert finished
INFO[0009] converted to "my.registry.com/repo:0.1-overlaybd", digest: "sha256:2446e61a1ea59a980b96e6e1e32b639ac4be4a05b64cb45ee2c8b4bd7f9c038e"
INFO[0009] overlaybd build finished

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #284

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[WaberZhuang]

LGTM

[liulanzheng]

@yuchen0cc pls see the e2e failure