Drop BuildKit cache mounts so legacy builder can build the image

The host runner's Docker CLI forces the buildx path when DOCKER_BUILDKIT=1
is set, but buildx isn't installed on the runner, so the build errored
before starting. Revert the env var and remove the two RUN --mount=type=cache
directives so the legacy builder can parse the Dockerfile. Normal layer
caching still applies; only the npm download cache reuse is lost.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/docker-build.yml

@@ -21,8 +21,6 @@ jobs:
           password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Build and Push
-        env:
-          DOCKER_BUILDKIT: "1"
         run: |
           # gitea.repository is already "owner/repo" (e.g. jason/breedr).
           IMAGE="registry.alwisp.com/${{ gitea.repository }}"

Dockerfile

@@ -10,7 +10,7 @@ COPY package.json package-lock.json ./
 COPY client/package.json client/package.json
 COPY server/package.json server/package.json
 COPY shared/package.json shared/package.json
-RUN --mount=type=cache,target=/root/.npm npm ci --no-audit --no-fund
+RUN npm ci --no-audit --no-fund
 
 FROM deps AS build
 COPY . .
@@ -23,7 +23,7 @@ COPY client/package.json client/package.json
 COPY server/package.json server/package.json
 COPY shared/package.json shared/package.json
 COPY server/prisma server/prisma
-RUN --mount=type=cache,target=/root/.npm npm ci --omit=dev --no-audit --no-fund
+RUN npm ci --omit=dev --no-audit --no-fund
 RUN npm run prisma:generate -w server
 RUN test -x /app/server/node_modules/.bin/prisma