docs: Use official curl image instead of appropriate/curl

[dmaciel]

The appropriate/curl last update is from two years ago:

$ docker run -it --entrypoint /bin/ash appropriate/curl:latest
$ curl --version
curl 7.59.0 (x86_64-alpine-linux-musl) libcurl/7.59.0 LibreSSL/2.6.3 zlib/1.2.11 libssh2/1.8.0
Release-Date: 2018-03-14
Protocols: dict file ftp ftps gopher http https imap imaps pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: AsynchDNS IPv6 Largefile NTLM NTLM_WB SSL libz UnixSockets HTTPS-proxy

There is already an official curl image at https://hub.docker.com/r/curlimages/curl (The link to the docker hub can be found in the official curl download page: https://curl.haxx.se/download.html)

$ docker run -it --entrypoint /bin/ash curlimages/curl:latest
$ curl --version
curl 7.70.0-DEV (x86_64-pc-linux-musl) libcurl/7.70.0-DEV OpenSSL/1.1.1d zlib/1.2.11 libssh2/1.9.0 nghttp2/1.40.0
Release-Date: [unreleased]
Protocols: dict file ftp ftps gopher http https imap imaps pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: AsynchDNS HTTP2 HTTPS-proxy IPv6 Largefile libz NTLM NTLM_WB SSL TLS-SRP UnixSockets

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Optional. My organization is added to USERS.md.
• I've signed the CLA and my build is green (troubleshooting builds).

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@a724574). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3695   +/-   ##
=========================================
  Coverage          ?   40.84%           
=========================================
  Files             ?      115           
  Lines             ?    17474           
  Branches          ?        0           
=========================================
  Hits              ?     7138           
  Misses            ?     9429           
  Partials          ?      907           

Impacted Files	Coverage Δ
cmd/argocd/commands/repo.go	0.00% <0.00%> (ø)
cmd/argocd-util/commands/projects.go	37.00% <0.00%> (ø)
cmd/argocd/commands/version.go	0.00% <0.00%> (ø)
server/badge/color.go	100.00% <0.00%> (ø)
cmd/argocd/commands/app_actions.go	0.00% <0.00%> (ø)
util/text/text.go	100.00% <0.00%> (ø)
util/dex/dex.go	81.08% <0.00%> (ø)
util/http/http.go	41.66% <0.00%> (ø)
util/webhook/webhook.go	56.45% <0.00%> (ø)
controller/metrics/transportwrapper.go	0.00% <0.00%> (ø)
... and 105 more

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[jannfis]

Absolutely makes sense. Much appreciated, thanks a lot!

LGTM.