fix: use COPY heredoc in GNOME 48-50 Dockerfiles (fixes broken image builds)

[jeevesforjoel]

Problem

The weekly image build CI has been failing for gnome-shell-48, gnome-shell-49, gnome-shell-49-x11, and gnome-shell-50 since they were added (run 29232568352):

ERROR: failed to build: failed to solve: dockerfile parse error on line 28:
unknown instruction: GSK_RENDERER=cairo

Root cause

All four Dockerfiles use the dockerfile:1.4 syntax directive, then write a wrapper script via a shell heredoc inside RUN:

RUN cat > /path <<'EOF'
#!/bin/sh
GSK_RENDERER=cairo ...
EOF

With the dockerfile:1.4 syntax parser active, the <<'EOF' inside a RUN command is interpreted as a Dockerfile-level heredoc, not a shell heredoc. The parser then reads #!/bin/sh and GSK_RENDERER=cairo as Dockerfile instructions, causing the parse error.

Fix

Replace RUN cat > /path <<'EOF' with COPY <<'EOF' /path — the correct Dockerfile 1.4 syntax for writing file content inline.

Also adds the missing # syntax=docker/dockerfile:1.4 directive to gnome-shell-49-x11.Dockerfile.

This unblocks the soft-brightness-plus E2E tests for GNOME 48/49/50, which pull these images from ghcr.io/jkitching/.