Revive external_custom target

[smlambert]

We had created a prototype under the Outreachy project called Tweetest, to be able to more easily extend external tests that meet a certain criteria (those that pull test material from a single repo, and use maven to run their tests).

Related: #2076

3 parameters were added to be able to trial out this idea (EXTERNAL_CUSTOM_REPO, EXTERNAL_CUSTOM_BRANCH and EXTERNAL_TEST_CMD which defaults to 'mvn clean install').

This issue is to revive the prototype and verify it works for some sample repositories. A Grinder job called Grinder_ext_custom has been created with the additional parameters. A recent run (Grinder_ext_custom/21) with the following parameter settings fails in the following way:

EXTERNAL_CUSTOM_REPO=https://github.com/radsz/jacop.git
EXTERNAL_CUSTOM_BRANCH=4.10.0
EXTERNAL_TEST_CMD=mvn --batch-mode clean install

12:09:05       [exec] #10 [6/9] COPY jacop/test.sh /test.sh
12:09:05       [exec] #10 ERROR: failed to calculate checksum of ref IOHW:E4VQ:KTS2:COOU:5DLG:NW4B:XV2Z:TFDL:SFRV:IF2C:7NZI:V65O::ppf2jo0abn8pypgbxintwpn3o: "/jacop/test.sh": not found
12:09:05       [exec] 
12:09:05       [exec] #4 [1/9] FROM docker.io/library/eclipse-temurin:11-jdk@sha256:bc9f393dff60292b0e7c10e2d8c5263f404672d5ada65eb38d416616d3593e31
12:09:05       [exec] #4 resolve docker.io/library/eclipse-temurin:11-jdk@sha256:bc9f393dff60292b0e7c10e2d8c5263f404672d5ada65eb38d416616d3593e31 0.1s done
12:09:05       [exec] #4 sha256:bc9f393dff60292b0e7c10e2d8c5263f404672d5ada65eb38d416616d3593e31 7.19kB / 7.19kB done
12:09:06       [exec] #4 CANCELED
12:09:06       [exec] ------
12:09:06       [exec]  > [6/9] COPY jacop/test.sh /test.sh:
12:09:06       [exec] ------
12:09:06       [exec] Dockerfile.hotspot.full:68
12:09:06       [exec] --------------------
12:09:06       [exec]   66 |     
12:09:06       [exec]   67 |     # This is the main script to run jacop tests
12:09:06       [exec]   68 | >>> COPY jacop/test.sh /test.sh
12:09:06       [exec]   69 |     COPY test_base_functions.sh test_base_functions.sh
12:09:06       [exec]   70 |     
12:09:06       [exec] --------------------
12:09:06       [exec] ERROR: failed to solve: failed to compute cache key: failed to calculate checksum of ref IOHW:E4VQ:KTS2:COOU:5DLG:NW4B:XV2Z:TFDL:SFRV:IF2C:7NZI:V65O::ppf2jo0abn8pypgbxintwpn3o: "/jacop/test.sh": not found
12:09:06  

Investigate the failure, and what recent changes to this area of code may have done to affect this target. Suspect we should be copying from external_custom/test.sh to /test.sh to be able to proceed.

[smlambert]

FYI @sophia-guo (so you are aware that there is a Grinder job to test these cases)

[sophia-guo]

The first thing I see is the parameter is not passed

12:09:05       [exec] #7 [3/9] RUN git clone
12:09:05       [exec] #7 CACHED