Add SHELL instruction to avoid JSONArgsRecommended warning

[Ivanov1ch]

• I have reviewed the OFRAK contributor guide and attest that this pull request is in accordance with it.

One sentence summary of this PR (This should go in the CHANGELOG!)
Added SHELL instruction to image building script to ensure consistent OS signal behavior
Link to Related Issue(s)
N/A
Please describe the changes in your request.
As build_image.py stands now, building images with ENTRYPOINTs including &, &&, etc leads to the following warning in the build log for finish.Dockerfile:
WARN: JSONArgsRecommended: JSON arguments recommended for ENTRYPOINT to prevent unintended behavior related to OS signals

Eg, when running make tutorial-image:

...
 > importing cache manifest from redballoonsecurity/ofrak/tutorial-base:master:
------
[+] Building 0.5s (17/17) FINISHED                                                                                                                                                                  docker:default
 => [internal] load build definition from finish.Dockerfile                                                                                                                                                   0.0s
 => => transferring dockerfile: 1.88kB                                                                                                                                                                        0.0s
 => WARN: JSONArgsRecommended: JSON arguments recommended for ENTRYPOINT to prevent unintended behavior related to OS signals (line 46)                                                                       0.0s
...

Or when building ofrak-ghidra.yml (as it currently stands in master):

...
 > importing cache manifest from redballoonsecurity/ofrak/ghidra-base:master:
------
[+] Building 0.5s (15/15) FINISHED                                                                                                                                                                  docker:default
 => [internal] load build definition from finish.Dockerfile                                                                                                                                                   0.0s
 => => transferring dockerfile: 1.46kB                                                                                                                                                                        0.0s
 => WARN: JSONArgsRecommended: JSON arguments recommended for ENTRYPOINT to prevent unintended behavior related to OS signals (line 38)                                                                       0.0s
...

As per the docs for this warning, one way to fix it is to pass the ENTRYPOINT arguments in exec form, but this wouldn't work for our ofrak-ghidra, for example, because exec form would not allow &. The simpler way of fixing this warning be to explicitly define the shell to be /bin/bash, which this PR does by adding SHELL ["/bin/bash", "-c"] before the ENTRYPOINT.
Anyone you think should look at this, specifically?
N/A

[alchzh]

Bit of an XY problem here. Why is ENTRYPOINT being used at all here? None of the "entrypoints" in the yaml configs behave like ENTRYPOINTs should, they're more like CMDs.

[Ivanov1ch]

Bit of an XY problem here. Why is ENTRYPOINT being used at all here? None of the "entrypoints" in the yaml configs behave like ENTRYPOINTs should, they're more like CMDs.

I feel that ENTRYPOINT remains more appropriate than CMD because we are often treating these containers as standalone executables, with maybe some extra args passed to enable CORS or import from some module. The docker blog agrees with using ENTRYPOINT for such a purpose.