Fix deprecated Dockerfile ENV format (go-gitea#31450)

See
https://docs.docker.com/reference/build-checks/legacy-key-value-format/.
Fixes these warnings seen during the docker build:

```
 4 warnings found (use --debug to expand):
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 5)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 9)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 75)
 - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 76)
 ```

Introduced in: moby/buildkit#4923

Dockerfile

@@ -2,11 +2,11 @@
 FROM docker.io/library/golang:1.22-alpine3.20 AS build-env
 
 ARG GOPROXY
-ENV GOPROXY ${GOPROXY:-direct}
+ENV GOPROXY=${GOPROXY:-direct}
 
 ARG GITEA_VERSION
 ARG TAGS="sqlite sqlite_unlock_notify"
-ENV TAGS "bindata timetzdata $TAGS"
+ENV TAGS="bindata timetzdata $TAGS"
 ARG CGO_EXTRA_CFLAGS
 
 # Build deps
@@ -72,8 +72,8 @@ RUN addgroup \
  git && \
  echo "git:*" | chpasswd -e
 
-ENV USER git
-ENV GITEA_CUSTOM /data/gitea
+ENV USER=git
+ENV GITEA_CUSTOM=/data/gitea
 
 VOLUME ["/data"]
 

Dockerfile.rootless

@@ -2,11 +2,11 @@
 FROM docker.io/library/golang:1.22-alpine3.20 AS build-env
 
 ARG GOPROXY
-ENV GOPROXY ${GOPROXY:-direct}
+ENV GOPROXY=${GOPROXY:-direct}
 
 ARG GITEA_VERSION
 ARG TAGS="sqlite sqlite_unlock_notify"
-ENV TAGS "bindata timetzdata $TAGS"
+ENV TAGS="bindata timetzdata $TAGS"
 ARG CGO_EXTRA_CFLAGS
 
 #Build deps
@@ -75,14 +75,14 @@ COPY --from=build-env /go/src/code.gitea.io/gitea/contrib/autocompletion/bash_au
 
 # git:git
 USER 1000:1000
-ENV GITEA_WORK_DIR /var/lib/gitea
-ENV GITEA_CUSTOM /var/lib/gitea/custom
-ENV GITEA_TEMP /tmp/gitea
-ENV TMPDIR /tmp/gitea
+ENV GITEA_WORK_DIR=/var/lib/gitea
+ENV GITEA_CUSTOM=/var/lib/gitea/custom
+ENV GITEA_TEMP=/tmp/gitea
+ENV TMPDIR=/tmp/gitea
 
 # TODO add to docs the ability to define the ini to load (useful to test and revert a config)
-ENV GITEA_APP_INI /etc/gitea/app.ini
-ENV HOME "/var/lib/gitea/git"
+ENV GITEA_APP_INI=/etc/gitea/app.ini
+ENV HOME="/var/lib/gitea/git"
 VOLUME ["/var/lib/gitea", "/etc/gitea"]
 WORKDIR /var/lib/gitea