Use ENV key=value in Dockerfile

[trivikr]

This will fix workflow failure

#21 ERROR: failed to push ***/bskyweb:ac9d910e1e77c559eff8b32cd8412335f41074f1: failed commit on ref "manifest-sha256:fd35dcfb18dd12d130322733b0d01dbc209a292dfb4941f2e90a8074c24e774f": unexpected status from PUT request to https://***/v2/bskyweb/manifests/sha256:fd35dcfb18dd12d130322733b0d01dbc209a292dfb4941f2e90a8074c24e774f: 403 Forbidden
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 > exporting to image:
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 1 warning found (use docker --debug to expand):
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 20)
ERROR: failed to solve: failed to push ***/bskyweb:ac9d910e1e77c559eff8b32cd8412335f41074f1: failed commit on ref "manifest-sha256:fd35dcfb18dd12d130322733b0d01dbc209a292dfb4941f2e90a8074c24e774f": unexpected status from PUT request to https://***/v2/bskyweb/manifests/sha256:fd35dcfb18dd12d130322733b0d01dbc209a292dfb4941f2e90a8074c24e774f: 403 Forbidden
Error: buildx failed with: ERROR: failed to solve: failed to push ***/bskyweb:ac9d910e1e77c559eff8b32cd8412335f41074f1: failed commit on ref "manifest-sha256:fd35dcfb18dd12d130322733b0d01dbc209a292dfb4941f2e90a8074c24e774f": unexpected status from PUT request to https://***/v2/bskyweb/manifests/sha256:fd35dcfb18dd12d130322733b0d01dbc209a292dfb4941f2e90a8074c24e774f: 403 Forbidden

Example: https://github.com/bluesky-social/social-app/actions/runs/11653248662/job/32445429993#step:7:1252