docs: :memo: Update installation doc

[krausemi]

Description

Within this PR the custom docker file for the sidecar has been updated as the one which is currently available is not working. Furthermore the spawning of zombie bash processes has been avoided by using tini as entrypoint.

Fixes: https://github.com/argoproj-labs/argocd-vault-plugin/issues/460

Checklist

Please make sure that your PR fulfills the following requirements:

• [x] Reviewed the guidelines for contributing to this repository
• [x] The commit message follows the Conventional Commits Guidelines.
• [x] Tests for the changes have been updated
• [x] Are you adding dependencies? If so, please run go mod tidy -compat=1.17 to ensure only the minimum is pulled in.
• [x] Docs have been added / updated
• [ ] Optional. My organization is added to USERS.md.

Type of Change

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] New tests
• [ ] Build/CI related changes
• [x] Documentation content changes
• [ ] Other (please describe)

Other information

[werne2j]

@krausemi can you sign-off on the commit for the DCO to pass? Thanks.

[krausemi]

@werne2j: done. Sorry for the little delay.

[codecov-commenter]

Codecov Report

Merging #485 (90a4580) into main (b587367) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #485   +/-   ##
=======================================
  Coverage   75.31%   75.31%           
=======================================
  Files          25       25           
  Lines        1689     1689           
=======================================
  Hits         1272     1272           
  Misses        323      323           
  Partials       94       94           

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