Add linux/amd64 platform specification to Terraform deployer Dockerfile

[girodav]

System tests using the Terraform deployer fail on M1 Macbooks, due to the change of architecture from intel/x86 to M1/ARM.

The installation of the google cloud SDK within the container fails with the following error:

#6 13.66 Preparing to unpack .../google-cloud-sdk_370.0.0-0_all.deb ...
#6 13.66 Unpacking google-cloud-sdk (370.0.0-0) ...
#6 22.80 Setting up google-cloud-sdk (370.0.0-0) ...
#6 22.81 qemu-x86_64: Could not open '/lib64/ld-linux-x86-64.so.2': No such file or directory
#6 22.81 dpkg: error processing package google-cloud-sdk (--configure):

The reason is that, since no platform version is specifically defined, an ARM-related image is downloaded, and they do not have that library (see also https://stackoverflow.com/a/71611002 for reference)

Forcing linux/amd64 as platform version in the Dockerfile fixes the issue.

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• Start Time: 2022-06-27T12:03:30.269+0000

• Duration: 29 min 17 sec

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (31/31)	💚
Files	66.071% (74/112)	👍
Classes	61.146% (96/157)	👍
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Lines	33.068% (2784/8419)	👍
Conditionals	100.0% (0/0)	💚

[mtojek]

Merged, thanks! I'm convinced that it will help many folks :)