Fix undefined vars build warnings in Dockerfile #1725

Job	Run time
Test frontend	29s
Lint	1m 12s
Test mcp-server on 3.8	8m 8s
Test mcp-server on 3.9	5m 49s
Test mcp-server on 3.10	8m 22s
Test mcp-server on 3.11	9m 20s
Test mcp-server on 3.12	9m 10s
Test mcp-client on 3.8	9m 4s
Test mcp-client on 3.9	8m 23s
Test mcp-client on 3.10	8m 30s
Test mcp-client on 3.11	8m 48s
Test mcp-client on 3.12	10m 6s
Test dashboard on 3.8	8m 56s
Test dashboard on 3.9	8m 54s
Test dashboard on 3.10	9m 47s
Test dashboard on 3.11	9m 57s
Test dashboard on 3.12	10m 21s
Test archivematica-common on 3.8	8m 32s
Test archivematica-common on 3.9	8m 13s
Test archivematica-common on 3.10	8m 2s
Test archivematica-common on 3.11	8m 25s
Test archivematica-common on 3.12	9m 31s
Test storage-service on 3.9	5m 52s
Test migrations on 3.9	5m 32s
	3h 9m 23s

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