contrib/docker: publish the Docker image to ghcr.io

[SChakravorti21]

Three changes:

1. f0df082: make it so that the tests that exercise standalone dbs can run with a pre-built image (making the clustered tests work with a pre-built image will require more thought).
2. 6306089: add a sanity check to the image's entrypoint - user must specify a dbname for the database to start.
3. c54421b: add a GitHub Actions workflow for publishing the image to the GitHub container registry (ghcr). This does the following:
   a. Build the image for both amd64 and arm64.
   b. "Merge" the manifests so that a docker pull can choose the best image for the native architecture, and publish to ghcr.
   c. Verify that the published images pass tests/contrib_docker.

Tested the publish workflow from my fork:

schakravorti@G0071XFCV7 comdb2 % docker run -d --name comdb2 ghcr.io/schakravorti21/comdb2:latest mydb
84585048748d722437d938a59a090a5af4659a91a6d8197885dc5b5aba839e30
schakravorti@G0071XFCV7 comdb2 % docker exec comdb2 cdb2sql mydb local "create table t(i int)"
schakravorti@G0071XFCV7 comdb2 % docker exec comdb2 cdb2sql mydb local "insert into t values(42)"
(rows inserted=1)
schakravorti@G0071XFCV7 comdb2 % docker exec comdb2 cdb2sql mydb local "select * from t"
(i=42)

[roborivers]

Cbuild submission: Error ⚠.
Regression testing: Success ✓.

The first 10 failing tests are:
sc_resume_logicalsc_generated **quarantined**
ssl_san
consumer_non_atomic_default_consumer_generated **quarantined**
ssl_set_cmd
ssl_prefer
ssl_dbname
sc_downgrade [timeout] **quarantined**
reco-ddlk-sql [timeout] **quarantined**

[roborivers]

Cbuild submission: Error ⚠.
Regression testing: Success ✓.

The first 10 failing tests are:
analyze_partial_index_off_generated [failed with core dumped] **quarantined**
analyze [failed with core dumped] **quarantined**
consumer_non_atomic_default_consumer_generated **quarantined**
sc_downgrade [timeout] **quarantined**
skipscan [timeout] **quarantined**